With the buffer having the contents shown below,
1. Enable flymake-mode.
2. Narrow from the top of the buffer down to the indicated comment.
3. Execute flymake-start.
You should get an error "error in process sentinel: Wrong type argument:
integer-or-marker-p, nil". I suspect perl-flymake has similar issues so you
might want to check that out as well.

Buffer contents:

\documentclass{article}
\begin{document}
0
a
b
c
% <----- End narrow
d
e
f
g
h
i
j{
\end{document}