The X-Files especially was pretty infamous for it, and I believe has been used as an example of what not to do when it comes to planning out your arc lol. Although to be fair the 90's were a very different time when it comes to tv, and obviously they could never have predicted fans one day purchasing the DVDs and scrutinising the show as a whole, as opposed to just sitting down to watch a creepy episode on a Sunday night or whenever. And plus the show went on for a bit too long towards the end anyway, so they had to constantly throw in new twists for the mytharc, which meant occasionally retconning what they had already written...