If I have many buffers open (284 at the moment) and if I run
dabbrev-expand (M-/) to expand the word under my buffers then it
searches through every buffer and takes an understandably long
time. If I mispelled the word fragment then it never has a hope of
finding it and I'd like to cancel the operation. But C-g does not work
for some reason and so I have to wait a good 5 to 10 seconds for it to
finish scanning all my buffers. This gets very frustrating after the
third or fourth time.
I don't see why C-g wouldn't work here. Could you apply the following
patch and see if the problem persists? (This is not a fix; it's an
attempt to diagnose the problem. As far as I know, quitting should not
be inhibited here.)