“\end\end” causes “TeX capacity exceeded”

Question

I know this is invalid LaTeX code, but the code \end\end results in the following error:

! TeX capacity exceeded, sorry [input stack size=5000].
\end #1->\csname end#1
                      \endcsname \@checkend {#1}\expandafter \endgroup \if@e...
l.1 \end\end

Why does this result in "TeX capacity exceeded" when this is simply invalid syntax?

Answer 1

It's not simply invalid syntax it is an infinite loop.

\end\end

is

\end{\end}

so it is the end environment call and it expands its argument

\end is more or less \csname end#1\endcsname so that \end{table} executes \endtable.

so

\csname end\end\endcsname

so we have to expand the \end and get

\csname end\csname end\endcsname\endcsname

which is

\csname end\end\endcsname

and we loop and so on 500 times until you fill the input stack, each time \endcsname is taken as the argument of \end and re-inserted.