Linux reserves some fraction of each partition's space, which can be used for writing only by the root user, or by programs. Regular users will be given a "disk full" message even though there is some unused space on a partition. This is so that important operations can still be carried out by root when a partition is close to full.
I was not aware of this fact until recently, and my home partition was getting close to full, with a few 100's of megs free. While I was not able to create new files from within Linux, ext2fsd continued to let me write to the partition from Windows. When the disk was really close to full, Linux (in particular, GNOME and the Evolution mail client) started behaving strangely. When I figured out the problem, I cleared up the partition, which fixed almost all issues. However, Evolution continued to behave strangely. My best guess is that Evolution's mail stores got corrupted (by Evolution itself) during this time. While this is probably due to a bug in Evolution, other programs may also behave strangely when faced with this situation.
It will be nice if the ext2fs driver can also do something similar to Linux, and disallow, or at least warn the user from writing into a partition when it is close to getting full.