> I have used edebug and error-on-quit to trace the culprit and find a
 > reproducible recipe. It turns out it has nothing to do with whitespace.
 >
 > 1. emacs -q
 > 2. M-x which-func-mode
 > 3. C-x C-f emacs-gnus-ticked-doc.patch
 >
[...]
 > In my experience, I have found which-func-mode to be intrusive and
 > unreliable. So I guess I will have to disable it to avoid further
 > trouble.

Looks to me like a bug in `diff-sanity-check-hunk' which fails to make
the hunk well-formed.  However, I think `diff-find-source-location'
shouldn't do any sanity checks in the first place when called with a
non-nil NOPROMPT argument.  So could you try whether the attached patch
solves your problem?

Thanks, martin.