Data's death
Data's died a few times, but the two that were supposed to stick were in Star Trek: Nemesis and then later in Star Trek: Picard's first season. The first time, we see Data die in a similar manner to Spock in Star Trek II: The Wrath of Khan. His death allows the Federation and its allies to defeat Picard's clone and the Remans who are aiding him. Later in the revival series Picard, Data is a memory of sorts that Picard visits every so often.
Eventually though, Picard has to let Data die a second time. Both times are supposed to be this emotionally interesting aspect but both times Data "dies" he's instantly brought back. In Nemesis he dies, only to be back in Picard's first season. Then he later dies again and is brought back once more through a mind-boggling dumb narrative exercise that sees Data, his siblings and lord knows who else, all merge into the same mind with Data somehow winning out.
It's stupid. Data's character was one of the best on The Next Generation but his plotlines were always so stupid. You know, it'd be ok if Data just died, right? You don't even need to replace him or anything. You had B4 and Lore right there. If you wanted to keep Data's actor around (Brent Spiner), you could've just done a continuation with B4 where he shows what he can be without his brother around.
It could've been interesting, and if not interesting on its own, it would've been more interesting than what we got. With us going back to the Data of it all once again.