One Baldur's Gate 3 player got the bad ending without even realizing it was the bad ending, missing out on the entirety of the massive Act 3. Naturally, I need to issue a spoiler warning through the end of Baldur's Gate 3 Act 2 here, so if you haven't yet reached the titular city, you should stop reading now.
Still here? Cool. Gale, one of the Baldur's Gate 3 companions, carries within him what amounts to a magical nuclear warhead, and as you develop your relationship with him, you learn that he's been charged with the destruction of the Absolute - the Elder Brain that's in control of the Mind Flayer tadpole in the heads of numerous people throughout the world, including your own. So he wants to get close to the Absolute and blow it up, taking himself and anyone nearby out with it.
There are a variety of reasons why that's a bad idea, but Gale does get the chance to execute his charge when you meet the Absolute at the end of Act 2. If you let him go through with the explosion, you get a unique ending, though admittedly a pretty anticlimactic one that leaves the world in a dire state. The narrator even concludes the scene by saying "It is an ending, of sorts. Though not the one destiny had in store for you."
A reasonable person might simply reload and make a different choice at that point. But in a post titled I'm so incredibly stupid, Reddit user megaderp675 defied common wisdom. "I don’t know if anyone else had this same thing happen to them but I realize now that I didn’t even see Act 3 and I thought I beat the game, but no, I never even went to the city. Instead I just suicide bombed Gale and rolled credits and thought Act 3 was that. Which is very wrong."
"I mean it’s still a valid ending, just not a good one," PStriker32 notes in the comments. There's a running theme in the comments that this fakeout ending actually makes sense from an in-universe perspective - you get a perfect opportunity to wipe out the biggest threat the world is currently facing, so why not take it? The fact that your death is anti-climactic is just a harsh dose of reality.
This comment from Ithinkillgrowthis is my favorite, though: "LOVE IT. I did this with Skyrim. Proudly told my husband I finally beat it after hundreds of hours of making new characters. He asked me what I thought of the dragon. …hm? What dragon? I crushed the Stormcloak rebellion. Turns out I didn’t know what the game was about."
Baldur's Gate 3 players have also recently been stunned by the revelation that you can shower the gore off.