So here's a story of, by far, the weirdest bug I've encountered in my CS career.

Along with @maciejwolczyk we've been training a neural network that learns how to play NetHack, an old roguelike game, that looks like in the screenshot. Recenlty, something unexpected happened. We use a model by @JensTuyls that clones expert behavior on NetHack, and we improve it using RL methods. That model gets 5000 points and we finetune it in the game so that the score improves. However, suddenly in a recent run, Jens' model only got 3000 points. Quite a drop.