The sense of achievement and relief over beating that final boss, solving a final puzzle or saving the world is one of unbridled joy. You seek validation, the thrill of a journey's end and a metaphorical pat on the back.
So when the game points a finger and laughs instead, it can really knock the wind out of your sails.
Imagine taking the time to hunt down a finite number of items around the world, only for your prize to be something you produce on a regular basis. Or when you think you've reached the end, and the game rug-pulls another several hours of gameplay out for you.
Even worse, when the whole point of your adventure has the indecency to be somewhere else when you get there, or doesn't remember what you've done for them.
There's been no end of false endings, joke endings and downright mockery for playing the game on easier difficulty settings over the years. It's even more insulting when the game essentially breaks the fourth wall to address you directly.
Some of these are end game mockeries, whilst others are when you venture off the beaten track for the thrill of the rewards. Which one bugged you the most?