Guide to LaTeX (4th Edition) (Tools and Techniques

416 Appendix C. Error Messages
! LaTeX Error: Bad \line or \vector argument.
The first argument of a \line or \vector command specifies the angle
of the line or arrow. This message states that the selected angle entries
are invalid. See pages 293–294.
! LaTeX Error: Bad math environment delimiter.
L AT E X has encountered a math switch command in the wrong mode: either
a \[ or \( command in math mode or the corresponding \] or \) in
normal text mode. Either the math switches have been improperly paired
or some braces {...} are incorrect.
! LaTeX Error: \begin{...} on input line ... ended by \end{...}.
L AT E X has encountered an \end command without a corresponding \begin
of the same name. This may be due to a typing mistake in the name of the
environment, or to the omission of a previous \end command. A good
way to avoid this error is always to enter the \end command immediately
after the \begin, inserting the actual environment text with the editor in
between the two. This is especially useful for long, nested environments.
It also reduces the risk of typing the environment name incorrectly in the
\end command.
! LaTeX Error: Can be used only in preamble.
Many commands may only be called within the preamble. These include
\documentclass, \usepackage, \nofiles, \includeonly, \makeindex,
\makeglossary, and several others. Certain commands that only have
meaning within class or package files, such as \ProvidesClass and
\ProvidesPackage (Section D.2.1), as well as many \Declare.. and
\Set.. commands are also only allowed in the preamble. If one of these
commands is issued after \begin{document}, this message is printed.
! LaTeX Error: Command ... invalid in math mode.
A command has been issued in math mode that only makes sense in
text mode, such as \item or \circle. The font declarations \itshape,
\bfseries, and so on, also produce this error in math mode, since the
math alphabet commands \mathit, \mathbf should be used instead.
! LaTeX Error: Command ... already defined.
The user has tried to redefine an existing structure with one
of \newenvironment, \newcommand, \newtheorem, \newsavebox,
\newfont, \newlength, \newcounter, or \DeclareMathAlphabet.
Either a different name must be selected or, in the case of commands
and environments, the \renew... version must be employed. (Note that
when an environment named sample is defined, the commands \sample
and \endsample are also created.)