Any Reason Why $_FILES['userfile']['error'] Wouldn't Be Populated?

I am in the middle of writing an image upload script and am limiting
the size of uploads via upload_max_filesize. This all seems to be
working well, and if I try and upload a bigger file then I get an
UPLOAD_ERR_INI_SIZE sat in $_FILES['userfile']['error']. All hunkey
dorey.

However, in my testing there are one or two large files that cause
problems. PHP appears to abort the upload early as it should, but when
my script runs, $_FILES isn't populated at all?!

I have tried to narrow down the difference in these file but I can't
seem to find anything definate. All the ones that fail are AVI or WMV
files if that gives any clue, but even if I rename them to MPG (others
of which work ok), and check the MIME type being sent up has also
changed to mpg, then they still fail as before. I have sent bigger and
smaller files all ok, so it's not size. I can't see anything in the
filename itself that would cause problems.