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Parsing -- gotta love it
In knee-jerk reaction to something that appeared on comp.compilers, I figured I'd go off and write a TeX file that defined, and invoked, a macro named \{} (that is, the name of the macro is \{}, not empty).
This has all kinds of terrible implications. Mostly they're only exploited by the LaTeX \makeatletter and \makeatother macros to "hide" things from non-package code. But nothing actually stops you from creating a macro named \$; if I understand the implications of \active, nothing stops you from creating a macro named just $ either and thereby confusing everybody who expects math mode.
% create a strangely-named macro
\expandafter\def\csname {}\endcsname{foo}
% invoke it
\catcode`\{=11
\catcode`\}=11
\{}
% switch back to sanity and quit
\catcode`\{=1
\catcode`\}=2
\bye
This has all kinds of terrible implications. Mostly they're only exploited by the LaTeX \makeatletter and \makeatother macros to "hide" things from non-package code. But nothing actually stops you from creating a macro named \$; if I understand the implications of \active, nothing stops you from creating a macro named just $ either and thereby confusing everybody who expects math mode.