I've got a TDC-3800 with DC-6525 tapes, and fsf'ing over a medium-size
(80-100MB) file makes mt return with EIO or something like that, before
the fast-forward is finished, and waiting a good while (sorry, didn't
time it)
fsf'ing over small files works fine.

After that, when the command finishes, I got a kernel error that was
along the lines of "CCB block already dequeued", then the SCSI bus _HUNG_,
use light ON, /usr/bin/login gaves I/O errors when I tried to log in, 
and eventually the machine had to be hard-reset.

This is with a PCI NCR SCSI controller, but it used to happen with
my ISA AHA-1542B, so I don't think it's controller-specific.

bsf'ing files is worse.

Simply reading them a block at a time works fine, of course.