### Capacity exceeded [semantic nest...]

    ! 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.