One of the most ironic scenes is when Vincent Vega, a hitman, is killed by Butch Coolidge, the boxer he was supposed to kill. Here, the hunter becomes the hunted in a very unexpected way. Vega, who was so confident in his own abilities and the situation, ended up being shot in the most ordinary of settings - a bathroom. It completely flips the power dynamic that was initially established between them.
Perhaps the scene where Butch saves Marsellus Wallace from being raped in the basement. Butch had every reason to leave Wallace there, as Wallace had tried to have him killed. But instead, Butch helps him. It's ironic that the man who was supposed to be his enemy becomes someone he rescues, highlighting the complex relationships and moral ambiguity in the movie.
The scene where Jules and Vincent are in the diner and Jules decides to give up his life of crime right after they've just had a violent encounter. It's ironic because they've been so immersed in this violent underworld, and then suddenly Jules has this moment of spiritual awakening. Just moments before, they were ready to kill, and now Jules is talking about walking away from it all. It shows how quickly things can change in their chaotic world.