Which are your "favourite" confusing error messages?

Question

This question: \addvspace can't replace \vspace shows a situation in which an error message can be confusing. If \addvspace is used when not in vmode the error message obtained is

! LaTeX Error: Something's wrong--perhaps a missing \item.

which can be seen, for example, compiling the following simple example

\documentclass{article}
\begin{document}
a\addvspace{3em}
\end{document}

in this case, the error message is not really descriptive and could be misleading. The definition of \addvspace (Section 16.5 Vertical spacing in source2e) explains what's going on:

\def\addvspace#1{%
  \ifvmode
    \if@minipage\else
      \ifdim \lastskip =\z@
        \vskip #1\relax
      \else
      \@tempskipb#1\relax
        \@xaddvskip
      \fi
    \fi
  \else
    \@noitemerr
  \fi}

so when \addvspace is used not in vmode, \@noitemerr is called producing the mentioned error message, as can be seeing in lterror.dtx.

\gdef\@noitemerr{%
<!autoload> \@latex@error{Something's wrong--perhaps a missing %
<!autoload> \protect\item}\@ehc}
<autoload> \@autoerr\@noitemerr}  

Which other situations have you encountered in which the error message obtained seems confusing and (apparently) not related to the problem originating the error?

I would suggest to accompany each answer with the textual error message, a minimal example code producing the error, and a brief explanation of why that is the error message.

Answer 1

There are many, I prefer the ones with a bit of humor from TeX itself, my two favorites:

Sorry, Pandora. (You sneaky devil.)

To trigger this message type:

\setbox0=\vbox{\halign{#\hfil\cr a\cr b\cr}}
\unhbox0
\bye

This will trigger an error Incompatible list can't be unboxed. Press 'h' for help and you will see the error.

and

Pretend that you’re Hercule Poirot: Examine all clues

How to trigger this message is left as an exercise for the reader. Clue is in the file TEX.POOL.

Answer 2

Partially taken from the TeX FAQ entry Capacity exceeded [semantic nest …]: The MWE

\documentclass{article}
\begin{document}
\def\silly{\hbox{here's \silly being executed}}
\silly% Be silly
\end{document}

produces the following error:

! TeX capacity exceeded, sorry [grouping levels=255].
<to be read again> 
                   {
l.4 \silly
          
If you really absolutely need more capacity,
you can ask a wizard to enlarge me.

Answer 3

Does an infinite loop count as confusing error message?

In some early experiments with catcodes I tried the following:

\documentclass{article}

\newcommand{\makeunderscoreactive}{\catcode`_=\active }
\newcommand{\makeunderscoreother}{\catcode`_=12\relax}
\makeunderscoreother
   \def\underscoreother{_}
\makeunderscoreactive
   \def_{\underscoreother}

\begin{document}
   \texttt{hello_world.txt} % this is fine
   $a_0$ % this causes an infinite loop
\end{document}

To be honest I am still not really sure why it's behaving that way.