! TeX capacity exceeded, sorry [semantic nest
size=100].
...
If you really absolutely need more capacity,
you can ask a wizard to enlarge me.
Even though TeX suggests (as always) that enlargement by a wizard
may help, this message usually results from a broken macro or bad
parameters to an otherwise working macro.
The "semantic nest" TeX talks about is the nesting of boxes within boxes. A stupid macro can provoke the error pretty easily:
\def\silly{\hbox{here's \silly being executed}}
\silly
The extended traceback
(see general advice on errors)
does help, though it does rather run on. In the case above,
the traceback consists of
\silly ->\hbox {
here's \silly being executed}
followed by 100 instances of
\silly ->\hbox {here's \silly
being executed}
The repeated lines are broken at exactly the offending macro; of
course the loop need not be as simple as this - if \silly calls
\dopy which boxes \silly, the effect is just the same and
alternate lines in the traceback are broken at alternate positions.