My favorite was back when CRT's were still actively used and we had this one customer who complained that their monitor would have squiggly and wavy lines on it randomly. We eventually stayed onsite the entire day and got to see it happen...we replaced the monitor and the next day, same problem.
I went onsite again and was trying to figure out why could possibly be going wrong when it started doing it again. After a few minutes I realized I could hear a humming noise and after tracking the noise to the opposite wall I asked, what's in that room?
Oh, that's our printing machine for T-Shirts. Apparently the damn thing covered the entire wall on the other side and was nothing but some big electrical magnet (600V power). Everytime they switched it on, the monitor went beserk but no one made the connection there.
Besides telling them to move the system out of the room (surprised the HDD never scrambled) I told them maybe it wasn't such a good idea for people to be in the room either..unless they got some lead drapes for that wall...