On my system, issuing
for file in *.JPG; do nice -19 display -resize 25% $file &; done
in a directory on an SMBFS-mount with some 80 files produces a number of processes which only run for a fraction of a second and then all get stuck:
PID TT STAT TIME COMMAND
2069 1 DNL 0:00,27 display -resize 25% 1.JPG
2070 1 DNL 0:00,26 display -resize 25% 2.JPG
2071 1 DNL 0:00,27 display -resize 25% 3.JPG
The processes cannot be killed, and all further file operations on the same mount, even umount -f, will get stuck, too.
Even worse, this problem even manages to make clean rebooting impossible, and I have to use reboot -q instead.
It might date back as far as 2005, see: