+utility assumes
+.Fl std
+even when it is not specified, but other implementations may not.
+.El
+.Ss "Warnings related to bad macro arguments"
+.Bl -ohang
+.It Sy "unterminated quoted argument"
+.Pq roff
+Macro arguments can be enclosed in double quote characters
+such that space characters and macro names contained in the quoted
+argument need not be escaped.
+The closing quote of the last argument of a macro can be omitted.
+However, omitting it is not recommended because it makes the code
+harder to read.
+.It Sy "duplicate argument"
+.Pq mdoc
+A
+.Ic \&Bd
+or
+.Ic \&Bl
+macro has more than one
+.Fl compact ,
+more than one
+.Fl offset ,
+or more than one
+.Fl width
+argument.
+All but the last instances of these arguments are ignored.
+.It Sy "skipping duplicate argument"
+.Pq mdoc
+An
+.Ic \&An
+macro has more than one
+.Fl split
+or
+.Fl nosplit
+argument.
+All but the first of these arguments are ignored.
+.It Sy "skipping duplicate display type"
+.Pq mdoc
+A
+.Ic \&Bd
+macro has more than one type argument; the first one is used.
+.It Sy "skipping duplicate list type"
+.Pq mdoc
+A
+.Ic \&Bl
+macro has more than one type argument; the first one is used.
+.It Sy "skipping -width argument"
+.Pq mdoc
+A
+.Ic \&Bl
+.Fl column ,
+.Fl diag ,
+.Fl ohang ,
+.Fl inset ,
+or
+.Fl item
+list has a
+.Fl width
+argument.
+That has no effect.
+.It Sy "unknown AT&T UNIX version"
+.Pq mdoc
+An
+.Ic \&At
+macro has an invalid argument.
+It is used verbatim, with
+.Qq "AT&T UNIX "
+prefixed to it.
+.It Sy "invalid content in Rs block"
+.Pq mdoc
+An
+.Ic \&Rs
+block contains plain text or non-% macros.
+The bogus content is left in the syntax tree.
+Formatting may be poor.
+.It Sy "invalid Boolean argument"
+.Pq mdoc
+An
+.Ic \&Sm
+macro has an argument other than
+.Cm on
+or
+.Cm off .
+The invalid argument is moved out of the macro, which leaves the macro
+empty, causing it to toggle the spacing mode.
+.It Sy "unknown font, skipping request"
+.Pq man
+A
+.Xr roff 7
+.Ic \&ft
+request has an invalid argument.
+.El
+.Ss "Warnings related to plain text"
+.Bl -ohang
+.It Sy "blank line in fill mode, using .sp"
+.Pq mdoc
+The meaning of blank input lines is only well-defined in non-fill mode:
+In fill mode, line breaks of text input lines are not supposed to be
+significant.
+However, for compatibility with groff, blank lines in fill mode
+are replaced with
+.Ic \&sp
+requests.
+.It Sy "tab in filled text"
+.Pq mdoc , man
+The meaning of tab characters is only well-defined in non-fill mode:
+In fill mode, whitespace is not supposed to be significant
+on text input lines.
+As an implementation dependent choice, tab characters on text lines
+are passed through to the formatters in any case.
+Given that the text before the tab character will be filled,
+it is hard to predict which tab stop position the tab will advance to.
+.It Sy "whitespace at end of input line"
+.Pq mdoc , man , roff
+Whitespace at the end of input lines is almost never semantically
+significant \(em but in the odd case where it might be, it is
+extremely confusing when reviewing and maintaining documents.
+.It Sy "bad comment style"
+.Pq roff
+Comment lines start with a dot, a backslash, and a double-quote character.
+The
+.Nm
+utility treats the line as a comment line even without the backslash,
+but leaving out the backslash might not be portable.
+.It Sy "invalid escape sequence"
+.Pq roff
+An escape sequence has an invalid opening argument delimiter, lacks the
+closing argument delimiter, or the argument has too few characters.
+If the argument is incomplete,
+.Ic \e*
+and
+.Ic \en
+expand to an empty string,
+.Ic \eB
+to the digit
+.Sq 0 ,
+and
+.Ic \ew
+to the length of the incomplete argument.
+All other invalid escape sequences are ignored.
+.It Sy "undefined string, using \(dq\(dq"
+.Pq roff
+If a string is used without being defined before,
+its value is implicitly set to the empty string.
+However, defining strings explicitly before use
+keeps the code more readable.
+.El
+.Ss "Errors related to equations"
+.Bl -inset -compact
+.It "unexpected equation scope closure"
+.It "equation scope open on exit"
+.It "overlapping equation scopes"
+.It "unexpected end of equation"
+.It "equation syntax error"
+.El
+.Ss "Errors related to tables"
+.Bl -inset -compact
+.It "bad table syntax"
+.It "bad table option"
+.It "bad table layout"
+.It "no table layout cells specified"
+.It "no table data cells specified"
+.It "ignore data in cell"
+.It "data block still open"
+.It "ignoring extra data cells"
+.El
+.Ss "Errors related to roff, mdoc, and man code"
+.Bl -ohang
+.It Sy "input stack limit exceeded, infinite loop?"
+.Pq roff
+Explicit recursion limits are implemented for the following features,
+in order to prevent infinite loops:
+.Bl -dash -compact
+.It
+expansion of nested escape sequences
+including expansion of strings and number registers,
+.It
+expansion of nested user-defined macros,
+.It
+and
+.Ic \&so
+file inclusion.
+.El
+When a limit is hit, the output is incorrect, typically losing
+some content, but the parser can continue.
+.It Sy "skipping bad character"
+.Pq mdoc , man , roff
+The input file contains a byte that is not a printable
+.Xr ascii 7
+character.
+The message mentions the character number.
+The offending byte is replaced with a question mark
+.Pq Sq \&? .
+Consider editing the input file to replace the byte with an ASCII
+transliteration of the intended character.
+.It Sy "skipping unknown macro"
+.Pq mdoc , man , roff
+The first identifier on a request or macro line is neither recognized as a
+.Xr roff 7
+request, nor as a user-defined macro, nor, respectively, as an
+.Xr mdoc 7
+or
+.Xr man 7
+macro.
+It may be mistyped or unsupported.
+The request or macro is discarded including its arguments.
+.It Sy "skipping item outside list"
+.Pq mdoc
+An
+.Ic \&It
+macro occurs outside any
+.Ic \&Bl
+list.
+It is discarded including its arguments.
+.It Sy "skipping column outside column list"
+.Pq mdoc
+A
+.Ic \&Ta
+macro occurs outside any
+.Ic \&Bl Fl column
+block.
+It is discarded including its arguments.
+.It Sy "skipping end of block that is not open"
+.Pq mdoc , man , eqn , tbl , roff
+Various syntax elements can only be used to explicitly close blocks
+that have previously been opened.
+An
+.Xr mdoc 7
+block closing macro, a
+.Xr man 7
+.Ic \&RE
+or
+.Ic \&UE
+macro, or the end of an equation, table, or
+.Xr roff 7
+conditional request is encountered but no matching block is open.
+The offending request or macro is discarded.
+.It Sy "inserting missing end of block"
+.Pq mdoc , tbl
+Various
+.Xr mdoc 7
+macros as well as tables require explicit closing by dedicated macros.
+A block that doesn't support bad nesting
+ends before all of its children are properly closed.
+The open child nodes are closed implicitly.
+.It Sy "scope open on exit"
+.Pq mdoc , man , eqn , tbl , roff
+At the end of the document, an explicit
+.Xr mdoc 7
+block, a
+.Xr man 7
+next-line scope or
+.Ic \&RS
+or
+.Ic \&UR
+block, an equation, table, or
+.Xr roff 7
+conditional or ignore block is still open.
+The open block is closed implicitly.
+.It Sy "escaped character not allowed in a name"
+.Pq roff
+Macro, string and register identifiers consist of printable,
+non-whitespace ASCII characters.
+Escape sequences and characters and strings expressed in terms of them
+cannot form part of a name.
+The first argument of an
+.Ic \&am ,
+.Ic \&as ,
+.Ic \&de ,
+.Ic \&ds ,
+.Ic \&nr ,
+or
+.Ic \&rr
+request, or any argument of an
+.Ic \&rm
+request, or the name of a request or user defined macro being called,
+is terminated by an escape sequence.
+In the cases of
+.Ic \&as ,
+.Ic \&ds ,
+and
+.Ic \&nr ,
+the request has no effect at all.
+In the cases of
+.Ic \&am ,
+.Ic \&de ,
+.Ic \&rr ,
+and
+.Ic \&rm ,
+what was parsed up to this point is used as the arguments to the request,
+and the rest of the input line is discarded including the escape sequence.
+When parsing for a request or a user-defined macro name to be called,
+only the escape sequence is discarded.
+The characters preceding it are used as the request or macro name,
+the characters following it are used as the arguments to the request or macro.
+.It Sy "argument count wrong"
+.Pq mdoc , man , roff
+The indicated request or macro has too few or too many arguments.
+The syntax tree will contain the wrong number of arguments as given.
+Formatting behaviour depends on the specific request or macro in question.
+Note that the same message may also occur as a WARNING, see above.
+.It Sy "missing list type, using -item"
+.Pq mdoc
+A
+.Ic \&Bl
+macro fails to specify the list type.
+.It Sy "missing manual name, using \(dq\(dq"
+.Pq mdoc
+The first call to
+.Ic \&Nm
+lacks the required argument.
+.It Sy "uname(3) system call failed, using UNKNOWN"
+.Pq mdoc
+The
+.Ic \&Os
+macro is called without arguments, and the
+.Xr uname 3
+system call failed.
+As a workaround,
+.Nm
+can be compiled with
+.Sm off
+.Fl D Cm OSNAME=\(dq\e\(dq Ar string Cm \e\(dq\(dq .
+.Sm on
+.It Sy "unknown standard specifier"
+.Pq mdoc
+An
+.Ic \&St
+macro has an unknown argument and is discarded.
+.It Sy "skipping request without numeric argument"
+.Pq roff
+An
+.Ic \&it
+request has a non-numeric or negative argument or no argument at all.
+The invalid request is ignored.
+.It Sy "skipping all arguments"
+.Pq mdoc , man , eqn , roff
+An
+.Xr mdoc 7
+.Ic \&Bt ,
+.Ic \&Ed ,
+.Ic \&Ef ,
+.Ic \&Ek ,
+.Ic \&El ,
+.Ic \&Re ,
+or
+.Ic \&Ud
+macro, an
+.Ic \&It
+macro in a list that don't support item heads, a
+.Xr man 7
+.Ic \&LP ,
+.Ic \&P ,
+or
+.Ic \&PP
+macro, an
+.Xr eqn 7
+.Ic \&EN
+macro, or a
+.Xr roff 7
+.Sq \&..
+block closing request is invoked with at least one argument.
+All arguments are ignored.
+.It Sy "skipping excess arguments"
+.Pq mdoc , roff
+The
+.Ic \&Bf
+macro is invoked with more than one argument, or a request of the
+.Ic \&de
+family is invoked with more than two arguments.
+The excess arguments are ignored.
+.El
+.Ss FATAL errors
+.Bl -ohang
+.It Sy "input too large"
+.Pq mdoc , man
+Currently,
+.Nm
+cannot handle input files larger than its arbitrary size limit
+of 2^31 bytes (2 Gigabytes).
+Since useful manuals are always small, this is not a problem in practice.
+Parsing is aborted as soon as the condition is detected.
+.It Sy "NOT IMPLEMENTED: Bd -file"
+.Pq mdoc
+For security reasons, the
+.Ic \&Bd
+macro does not support the
+.Fl file
+argument.
+By requesting the inclusion of a sensitive file, a malicious document
+might otherwise trick a privileged user into inadvertently displaying
+the file on the screen, revealing the file content to bystanders.
+The parser exits immediately.
+.It Sy "NOT IMPLEMENTED: .so with absolute path or \(dq..\(dq"
+.Pq roff
+For security reasons,
+.Nm
+allows
+.Ic \&so
+file inclusion requests only with relative paths
+and only without ascending to any parent directory.
+By requesting the inclusion of a sensitive file, a malicious document
+might otherwise trick a privileged user into inadvertently displaying
+the file on the screen, revealing the file content to bystanders.
+The parser exits immediately.
+.It Sy ".so request failed"
+.Pq roff
+Servicing a
+.Ic \&so
+request requires reading an external file.
+While trying to do so, an
+.Xr open 2 ,
+.Xr stat 2 ,
+or
+.Xr read 2
+system call failed.
+The parser exits immediately.
+Before showing this message,
+.Nm
+always shows another message explaining why the system call failed.
+.El