Flickering lights
In a previous life I installed a dozen TVs up on the walls around the office displaying various monitoring information running via HDMI over Cat5 adapters from a PC 30 metres away in the comms room. I couldn't work out for ages why the TVs would randomly go blank for a few seconds then back on again until I stayed late one evening and found they worked perfectly when the office was empty. Turned out to be interference from the IR motion detectors that kept the lights on in the office was somehow inducing enough noise into the Cat5 to upset the HDMI signal when people moved! Shielded Cat 6 rather expensively fixed the problem.
Biting the hand that feeds IT
