I was watching some old episodes of "The Young Ones" on NetFlix a while back, and noticed that they did this in their show quite a lot - each episode had at least one or two images inserted randomly in them for a split second. Nothing to do with the episode story at all.

I was intrigued because I never noticed them decades ago when watching the original series - they are far more obvious to me now. Must have been a common technique at the time, because I remember watching another English show from the same era (the name of the show escapes me), which did the same thing.