Unkillable zombie sshd-s?

[Ivan Voras]

I found out today that I have two zombie sshd processes on a busy server 
(dozen or so users over ssh, many other services), and I can't kill them.

sshd    38653  0.0  0.0     0    0  ??  Z    Mon08AM   0:00.03 <defunct>
sshd    75851  0.0  0.0     0    0  ??  Z     7:33PM   0:00.08 <defunct>

kill -9 doesn't complain, but doesn't do the job either.

Is there a way to find out what has happened and why does the situation 
occur? (I can't reboot the server for testing)