Fun: How can we generate funny/unexpected messages in LaTeX?

Question

Knuth says in "Chapter 6: Running TeX" of the TeXbook:

Error messages can be terrifying when you aren't prepared for them; but they can be fun when you have the right attitude. Just remember that you really haven't hurt the computer's feelings, and that nobody will hold the errors against you. Then you'll find that running TeX might actually be a creative experience instead of something to dread.

How can we generate funny/unexpected messages in LaTeX?

Here are my two cents:

\documentclass{article}
\newenvironment{foobar}{}{}
\begin{document}
\begin{foobar}
\end{\csname @currenvir\endcsname}
\end{document}

yields:

! LaTeX Error: \begin{foobar} on input line 7 ended by \end{foobar}.

The message comes into being as follows:

Carrying out the \begin-macro—without expanding them—triggers defining a macro \@currenvir from the tokens that form its argument.
Carrying out the \end-macro—without expanding them—by calling the kernel-macro \@checkend{⟨tokens that form the argument of the \end-macro⟩}—triggers defining a macro \reserved@a from the tokens that form its argument whereafter \reserved@a and \@currenvir get compared via \ifx.

If that comparison yields the \else-branch, then via \@badend{⟨tokens that form the argument of the \end-macro⟩} the error-message ! LaTeX Error: \begin{⟨tokens coming from expanding \@currenvir which in turn was defined from the tokens that form the argumment of the \begin-macro⟩} on input line 7 ended by \end{⟨tokens that form the argument of the \end-macro⟩} is delivered.
In the scenario above the set of tokens forming the argument of the \begin-macro differs from the set of tokens forming the argument of the \end-macro. Therefore the \ifx-comparison, which is triggered by \@checkend, between \@currenvir, whose definition comes from the tokens forming the argument of the \begin-macro, and \reserved@a, whose definition comes from the tokens forming the argument of the \end-macro, yields delivering the error-message via \@badend, whereby ⟨tokens coming from expanding \@currenvir which in turn was defined from the tokens that form the argumment of the \begin-macro⟩ and ⟨tokens that form the argument of the \end-macro⟩ expand to the same textual phrase.

\documentclass{article}
\let\elsecopy\else\AtEndDocument{\def\else{\let\else\elsecopy}}
\begin{document}
\end{document}

yields the following error-message:

! LaTeX Error: \begin{document} ended by \end{document}.

In this example, too, you stumble over the \ifx-comparison done by \@checkend when tracking things down:

The \else of the \ifx-comparison done by \@checkend for finding out if the \begin- and the \end-macro were used with the same argument is not taken for something that introduces the branch for the case of the \ifx-condition not being fulfilled.

More funny examples are appreciated. :-)

One day of the year, I would certainly upvote this question. Unfortunately it has come 9 months early… — Peter LeFanu Lumsdaine, Jul 01 '21 at 20:36
This could be useful by explaining what is the problem as egreg does. — projetmbc, Jul 02 '21 at 09:10
I don’t know anything about funny error messages. But once, when I was doing a very boring physics report with a fellow student, I did \let\oldpar=\par \def\par{\oldpar And physics sucks.\oldpar}. — Gaussler, Jul 02 '21 at 09:48
@Gaussler Nice. :-) Why did you use \oldpar and not \endgraf (or \@@par) ? — Ulrich Diez, Jul 02 '21 at 11:18
@UlrichDiez I actually didn’t really know either command. Never needed them before. ;-) — Gaussler, Jul 02 '21 at 11:21
@Gaussler I asked because as far as TeX/LaTeX goes, I often overlook subtle reasons for the ways people implement things.From answers to questions like "why did you implement things this way and not another" I learned a lot about the subtleties of (La)TeX. — Ulrich Diez, Jul 02 '21 at 11:26
I thought this was going to be "how can we make the error messages more meaningful?" — Teepeemm, Jul 02 '21 at 11:27
@UlrichDiez Cool ;-). I have often been thinking of creating a topic about how you can tease your LaTeX collaborators during group projects. Examples could be stuff like what I just mentioned, but also e.g. LuaTeX replacement of their names by Donald Duck or whatever. — Gaussler, Jul 02 '21 at 12:00
@Teepeemm For clarification I edited the subject/title of this question... :-) — Ulrich Diez, Jul 02 '21 at 19:47
Somewhat related/crosslink: errors - Interwoven alignment preambles are not allowed - what exactly does this mean? - TeX - LaTeX Stack Exchange — user202729, Feb 07 '23 at 17:35

score 20 · Accepted Answer · answered Jul 01 '21 at 22:11

> pdflatex funmessage
This is pdfTeX, Version 3.141592653-2.6-1.40.22 (TeX Live 2021) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./funmessage.tex
LaTeX2e <2021-06-01> patch level 1
L3 programming layer <2021-06-18>
(/usr/local/texlive/2021/texmf-dist/tex/latex/base/article.cls
Document Class: article 2021/02/12 v1.4n Standard LaTeX document class
(/usr/local/texlive/2021/texmf-dist/tex/latex/base/size10.clo))

! LaTeX Error: Missing \begin{document}.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...

l.3 \begin{document}

Note that the compiler stops at \begin{document} saying it's missing.

How is this obtained? Simple!

\documentclass{article}
\renewcommand{\fi}{fi}
\begin{document}
Foo
\end{document}

Never do \renewcommand on commands you don't know about.

Fun: How can we generate funny/unexpected messages in LaTeX?

1 Answers1

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