I'm curious how many episodes throughout Star Trek, from an in-universe perspective, either didn't happen or may as well not have happened, as in:
- All the events are forgotten (like in Voyager's "Unforgettable")
- Events are erased from the timeline (like that time loop episode in The Next Generation where Data sends the number 3 into the next loop)
- Events actually happened to duplicates of the main characters (like the Voyager episode on a duplicate ship)
A little clarification: An episode "didn't happen" if, by the end of it, all or most of the events are set back to a point where they effectively didn't happen (or arc for that matter) usually due to the resolution. It doesn't count as "didn't happen" if the situation is resolved but the events just happened to occur in a different universe so that they don't matter.
This last bit is important because it excludes "Inner Light", "Parallels", and mirror universe stories for the most part. (I forget if there are time travel ones.)