The 1984 movie The Terminator by James Cameron was a big hit in the 80's. While sci-fi movies with killer robots was nothing new (them going back probably at least to the 50's or even earlier), this was a movie that got really popular, and which transcended other sillier movies of the genre. It was gritty, it was thrilling, and it's an integral part of not only the 1980's cinematography but also culture.
Probably everybody knows the basic premise (and with this kind of movie it's probably rather useless to warn about spoilers): Nigh indestructible and unstoppable killer robot travels back in time from the future in order to kill the mother of the leader of the human resistance. The resistance also sends back a soldier, Kyle Reese, to protect the woman from the terminator. The film is stock full of grand scale chase sequences, and enormous amounts of violence and explosions (iow. it's one of the most iconic action thrillers of movie history.)
The killer robot, a "terminator", was played by Arnold Schwarzenegger, and it's by far one of his most iconic and memorable roles. If you don't know Arnold from any other role, you most probably know him at the very least from this one.
When the sequel came around in 1991, there was quite a lot of hype about it. Basically nobody could avoid hearing or reading about Arnold's new role in it: A good terminator. A terminator that now comes back in time to protect, rather than to kill.
And that was, in fact, a huge, huge spoiler... which basically nobody noticed. Not back then at least.
You see, Terminator 2 actually starts as if the same basic premise happens as in the first movie. It starts very similarly: It looks like an Arnold-looking terminator again travels from the future to the present and starts looking for John Connor, the future leader of the resistance. It also shows another man arriving. The movie sets it up to look like this man is another "Kyle Reese" sent this time to protect John Connor.
A good chunk of the beginning of the movie keeps their actual roles completely ambiguous. Someone who has seen the first movie, and has not been spoiled in any way, very easily gets the impression that the Arnold-looking terminator is the villain once again.
This ends up in a climactic scene where both the Arnold-looking terminator and the other guy find the young Connor at the same time, and it looks like the terminator is going to shoot him... And here is where the reveal, the twist, happens: It turns out that it was the other guy who was trying to kill him, and the Arnold-terminator is actually protecting him.
This is clearly intended to be a surprising twist... which ended up basically surprising nobody, because everybody had been spoiled. And curiously, nobody realized that it was a spoiler, and that this was supposed to be a surprise plot twist. (The reason for this is that the beginning is so ambiguous that it works the other way too. In other words, if you know in advance that Arnold-terminator is the "good guy", the beginning still makes perfect sense. Thus it doesn't feel odd and there isn't anything out of ordinary.)
It was a huge twist that basically everybody missed. Because everybody was spoiled by the hype and the advertising campaigns.