If it's a one off, then don't worry too much about it.
However, if it keeps happening then we'll need to know more about your system and how you use your machine to be able to help further. We'll need to know stuff like:
What type of machine it is?
What OS is it running?
What third party apps are running at the time, including theme changers and system hacks?
What third party hardware is connected?
What brand of printer is it, and how is it connected?