Tobacco smoke
Back in the '90s , I was working for a small consultancy that had a large UK Tobacco company as a client. This being the 90s. the tobacco company gave staff free cigarettes, and had absolutely no problem with staff smoking at their desks.
One day, I had to go on-site to deal with problems. The whole place stank of nicotine; and when I found the misbehaving PC and opened it up, there was half an inch of fag ash all over the boards, which explained the overheating and behaiour.
I suspect every PC in the building was like that.
Biting the hand that feeds IT
