When the the third Matrix movie came out, on opening day, the morning show on one local station started and ended pretty much every segment by spoiling the ending. They kept saying things like "For those just tuning in, we are just talking about how surprising it was that they killed off [Character Name] at the end of part 3" or "Okay were going to commercial, still shocked they killed [Character Name] at the end of the new movie." So when I went to see the movie I was constantly thinking about whether or not these guy really did spoil the movie as a joke, or if they gave a fake spoiler as a joke. Either way it was jerk move and the station got a lot of angry letters and emails about it.
For a while my wife was really into the show Masked Singer. If you don't know, it's a singing competition show where the contestants are all celebrities but wear elaborate costumes to hide their identity. Each episode a singer is eliminated and unmasked until the final episode where one person is declared the winner and then unmasked. A big part of the fun is trying to figure out who everyone is under the mask. My wife had seen an episode at a friend's house and really enjoyed it, but this was like season 3, we went back so she could binge watch the series from season 1. About 3 or 4 episodes in there was one singer whose voice sounded very familiar, my daughter and I were discussing them and we both agreed on who it might be. However my daughter was not fully convinced, fortunately our guess was someone who had been for singing in other media, so there were a number of youtube clips that showed this person singing that we could watch and try to compare. When my daughter did a search for this celebrity singing the first several search results were all about how this person won season 1 of Masked Singer. So not only was their identity spoiled for us but also the outcome of that season.
The big one for me however was Terminator, and Terminator 2. When the first Terminator came out I saw the trailers and desperately wanted to see the movie. As I was too young to go see an R rated film I had to wait until it came out on VHS. In the meantime it was getting good reviews and I even got to see a bit of it when it was reviewed by Siskel and Ebert. It was cool at first because they were talking about the movie and showing clips that weren't in the trailers. Unfortunately they chose to show the clip from the end of the movie where the semi truck explodes and the Terminators true form is revealed. I'm sure that I would have been tricked into that false sense of security thinking "Yeah! They got him he's finally defeated... wait WHAT?" but sadly I knew what was coming because I watched a review.
Now when Terminator 2 came along I was able to see it in theaters opening day. The problem was that all the trailers and the entire advertising campaign was centered around telling the audience "Hey, remember how in the first movie Arnie was the bad guy? Well now he's the hero!!" The entire opening act was devoid of tension because everyone in the theater already knew who to root for.