Intermittent problem with unencodable-char-position

[Harald Hanche-Olsen <hanche@math.ntnu.no>]

Evaluating the form

(unencodable-char-position 0 5 'iso-latin-1-unix 1 "100 Ω")

normally returns the list (4), since capital Omega is not encodable in
latin-1. However, after I have run emacs for a while, it happens that
this form begins to return nil [*]. I have no idea what triggers this
behaviour, and the only cure seems to be to quit and restart emacs.

I suspect some internal memory corruption, but if anyone here can
suggest another possible reason, I'd like to hear about it. Or if you
can think of a debugging technique that might shed some light on this,
I'll be happy to try it when it happens again. (I warn you that I run
on OS X, though, so debugging is, um, different.)

[*] I notice because attempts to save a buffer containing non-latin-1
characters with the latin-1 charset fails without the usual offer to
select a different character set. I have narrowed the problem down to
the above behaviour inside select-safe-coding-system-interactively.
(That code doesn't use the string argument, but it happens whether you
look at a string or the current buffer.)

- Harald