\protect
Footnotes, line breaks, any command that has an optional argument, and
many more are so-called fragile commands. When a fragile
command is used in certain contexts, called moving arguments, it
must be preceded by \protect
. In addition, any fragile
commands within the arguments must have their own \protect
.
Some examples of moving arguments are \caption
(see figure), \thanks
(see \maketitle), and
expressions in tabular
and array
environments
(see tabular).
Commands which are not fragile are called robust. They must not
be preceded by \protect
.
See also:
http://www-h.eng.cam.ac.uk/help/tpl/textprocessing/teTeX/latex/latex2e-html/fragile.html
http://www.tex.ac.uk/cgi-bin/texfaq2html?label=protect