This isn't much of a bug report, but perhaps it will help.
I sometimes get this error when I quit ediff. I use a standalone
minibuffer and pop-up-frames = t. I use these ediff settings:
(setq-default ediff-ignore-similar-regions t)
(setq-default ediff-auto-refine-limit 10000)
(setq ediff-grab-mouse 'maybe)
The buffers I compare (with `ediff-buffers') are in separate
frames. The ediff control panel is in a separate (tiny) frame.
Here is the backtrace I get:
Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)
balance-windows()
ediff-cleanup-mess()
run-hooks(ediff-quit-hook)
ediff-really-quit(nil)
ediff-quit(nil)
call-interactively(ediff-quit)
HTH.
In GNU Emacs 22.0.50.1 (i386-msvc-nt5.1.2600)
of 2006-07-19 on BOS-CTHEWLAP2