what could "use up" X forwarding connections?

[Tom Horsley]

Anyone have any clues about what could "use up" some resource
used to create a forwarded X connection (ssh -X)?

I've got these testbeds that run 24/7 on random collections
of both real and virtual machines, and they are all started
from inside a VNC session using ssh -X to get to the target
test machine. The theory is that any test which needs a X
server will be happy with the VNC server.

After running for a few months, tests sometime start failing
because they can't open the X server. Reboot the host (which
is running fedora 13) and recreate the VNC server and things
work again.

Seems like something is getting "used up" that the reboot
cleans up, but I haven't been able to find out what
that something is :-).