Tv show that you never watch..but then when you watch it you've already seen that episode?

why is it that when you 'never' watch a show, and then you when decide to watch it, its always an episode you've already seen? what's up with that? and why does it always seem to happen when there is nothing else to watch so you end up having to watch it again? its crazy and its spooky...is there a scientific explaination or mathematical equation to explain this?