Please try to open the attached file in emacs -Q, and after moving point
to the beginning of the 3rd line, type 'M-q'.  When debug-on-error is t,
it fails with such backtrace:

Debugger entered--Lisp error: (error "Invalid search bound (wrong side of point)")
  re-search-backward("[ \11]\\|\\c|.\\|.\\c|" 280 0)
  fill-move-to-break-point(280)
  fill-region-as-paragraph(184 210 nil nil 186)
  fill-comment-paragraph(nil)
  fill-paragraph(nil t)
  funcall-interactively(fill-paragraph nil t)
  command-execute(fill-paragraph)

BTW, evaluating debug-on-error in *scratch* shows t by default,
but 'C-h v debug-on-error' shows it's nil actually, strange.