Interestingly, as I pasted the code into this `message-mode' buffer,
`show-paren-mode' doesn't fail any more, since ";;" don't register as
comments any more. Maybe `scan-sexps' could be modified to scan
differently (i.e. ignoring comments) if the starting position is in a
comment. This would be preferable to just ignoring the scan errors like
the attached patch does.

What do you think?

    Oleh