Can delaying the reboot after a Windows XP automatic update installation cause unusual behavior?

"Over-keen cleaner popping into the room with the servers, unplugging the hub/switch (accidentally) in order to plug in his/her vacuum?

User pulling out a network cable to plug in a laptop?

Some piece of old tat which is only used occasionally (e.g. a cheque printer) which throws out loads of interference which takes down the network (or part of it)?

Mice?

Rats?

Badgers?

UPS tests which take down circuits which run your switch?

Old hard disks which are on the way out?

SAN controllers which sulk every third Wednesday?

I think a bit more info on times of day, frequency of problem, and so on, might help."