My Debian "Squeeze" now hangs for about 20 seconds (usually never does) at the display message "Asking all remaining processes to terminate". Then another message appears: "Currently running processes (pstree):" and it hangs for about 6 seconds. Then messages "killall5 can't get program name from /proc/xxxx", xxxx standing for pid number, presumably. About a dozen of these lines appear, then Failure in red. After all that the shutdown continues for a few seconds and is completed.
The messages go by so fast that I cannot get all the numbers, but the ones that I read are not in /proc or ps or anywhere else that I can think of.
Can anyone explain this odd behavior? I would like to get back to normal.

These unrecognized PIDs and attendant interruption of the shutdown process continued for many weeks but then one day they disappeared, so everything is back to normal.
Except: I have no understanding of why that was happening.