I thought YouTube had introduced a weird glitch to a video I uploaded, then checked my source and realised it was there too. Transitioning between two clips (fade to/from black) this rectangular box flashes up for a split second in two places like a blocky bolt of lightning and then is gone… probably to slope off and torment a newly wed couple with small children who have just moved into a conspicuously spooky mansion expecting to live happily ever after. 
The video was unlisted and now deleted as I need to make some more changes, otherwise I’d link to it.