8. "The Princess Is In Another Castle!"
You thought it was over? Not yet.
We've all been there - the game makes it abundantly clear it's entering the final stretch, and after doing battle with the "final" boss, it turns out there's still a little (or a lot) game left to go.
The most famous example is of course the whole "Our princess is in another castle!" trope from the Mario series, where Mario repeatedly learns that he's been fighting a decoy version of Bowser, and Princess Peach is actually sequestered with the real Bowser at another castle entirely.
Resident Evil 2 also has a fantastic example, when after defeating the Tyrant, it makes one last attempt to kill you on the emergency escape train out of Raccoon City just as it all looked safe and done.
While it can certainly prove frustrating in bad games, if it's a game you're enjoying, it simply means you've got more game to sink your teeth into, which is rarely a bad thing.