Looking at the frequent buildbot failures, they always seem to happen in
TestMaildir. Therefore, it may have to do with the metadata-updating
behaviour of the filesystem (since maildir uses one file per message).

Unfortunately, while I tried to reproduce the failures on my home system
with a variety of filesystems (ext3, ext4, reiserfs, xfs and even vfat),
I never got any failure here.

Ezio, could you give details on your system, your filesystem and its
mount options? (if you don't know them, look in /proc/mounts)