The mystery of the internet that only worked when in the dark.
I used to work on a service desk where we'd support a large chain of pharmacy stores in the UK, we had many such stories such as keyboards that only started working when you'd first flipped them upside down (The trick being that the cable in the back would come loose, and doing this pushed it back into position).
However the weirdest call I experienced was a store who told me, their internet only ever worked if they sat in darkness in the back office. The moment they turned the lights on, the internet would break. I have no idea how they originally discovered this fact, but from much testing it turned out to be true - turning the lights on in the back office caused the 56k modem to completely drop the line.
We swapped out all the equipment, part by part, wondering what on earth was going on, the modem was fine, as was the computer itself, the entire network in fact being faultless other than this very strange behaviour.
Eventually we had to call out BT to come and look at the phone lines, and for a few weeks they kept coming back after making changes to the phone line but no one could explain what was happening. In the end we had a higher up engineer turn up who had some EE expertise. He suggested they replace the bulbs and ballast in the florescent lamps they had in their back office. Instantly the problem was resolved! I'm not sure of the exact details, but basically the lights coming on was creating a charge on the phone line, large enough that it caused disconnections each time the lights were turned on.