This may seem like an obvious one, but try to leave time to playtest your game front to back once you think you’re finished. For example, in my game, I wanted to fix a bug for the win conditions, so I fixed it in one scene and pasted it into the others. I had forgotten that this code included the changing from one scene to the next when you win, and I had copied this from the final level, so now in the GDGames version it’ll bring you back to the menu after only the first level; wouldn’t have happened if I had playtested that level from that point and not just the third one.
This is so important. Repeatedly playing through the game (and purposefully dying at times) saved me in the end. At one point the boss spawned every time I entered that room, which was definitely not ideal. I think a lot of us forget to play the game as someone who doesn't know the 'correct' path and we miss how others will experience the game.