I'm running Ubuntu Squeeze and on one of the partitions df is showing the Total size as 335G:
Filesystem Size Used Avail Use% Mounted on /dev/sdb 335G 225G 94G 71% /mnt
However in the past it was showing as 360GB (which is the actual size):
fdisk -l /dev/sdb Disk /dev/sdb: 365.0 GB, 365041287168 bytes
lsof +L1 does not return anything (and anyway if this would be the case the Total space should not be affected.)
On this partition I'm writing (and deleting) a lot of files and this happened before in the past, but problem solved by itself.