I'm not sure how to replicate, buy there seems to be a bug where if you're holding one of the movement keys while leveling up, one of your choices will disappear.
Also, pressing escape while the game is in fullscreen, (which the game defaults to), exits fullscreen without pausing the game.
Kkeeping LMB held while the level up items appear may cause you to immediately chose one you don't intend to. Since you're most likely to be holding LMB throughout the entire game, this happened to me quite often.
Other than that, it's a pretty frantic twin-stick shooter, (well, top-down KBM), like a non-stop Smash TV!