An extra }?

You’ve looked at your LaTeX source and there’s no sign of a misplaced } on the line in question.

Well, no: this is TeX’s cryptic way of hinting that you’ve put a fragile command in a moving argument.

For example, \footnote is fragile, and if we put that in the moving argument of a \section command, as

\section{Mumble\footnote{I couldn't think of anything better}}

we get told

! Argument of \@sect has an extra }.

The same happens with captions (the following is a simplification of a comp.text.tex post):

\caption{Energy: \[e=mc^2\]}

giving us the error message

! Argument of \@caption has an extra }.

The similar (but more sensible):

\caption{Energy: \(e=mc^2\)}

is more tiresome, still: there’s no error when you first run the job … but there is on the second pass, when the list of figures (or tables) is generated, giving:

! LaTeX Error: Bad math environment delimiter.

in the \listoffigures processing.

The solution is usually to use a robust command in place of the one you are using, or to force your command to be robust by prefixing it with \protect, which in the \section case would show as

\section{Mumble\protect\footnote{I couldn't think of anything better}}

However, in both the \section case and the \caption case, you can separate the moving argument, as in \section[moving]{static}; this gives us another standard route — simply to omit (or otherwise sanitise) the fragile command in the moving argument. So, one might rewrite the \caption example as:

\caption[Energy: (Einstein's equation)]{Energy: \(E=mc^2\)}

In practice, inserting mathematics in a moving argument has already been addressed in LaTeX2e by the robust command \ensuremath:

\caption{Energy: \ensuremath{E=mc^2}}

So: always look for alternatives to the \protect route.

Footnotes can be even more complex; “footnotes in LaTeX section headings” deals specifically with that issue.

FAQ ID: Q-extrabrace