There I am with an audience in front of me waiting to be delighted with a demo of some online magic when I find that I can't log onto the PC, which tells me my PC account is disabled. I ring the helpdesk: they can't fathom why either.
An hour later it turns out that it isn't my account that's disabled, it's the PC. Apparently some of our friends in IT have decided that if a PC hasn't been used for a few weeks they'll disable it on the network. But not bother telling anyone.