I liked your demo. I ran into 3 bugs from what I've noticed. My first involved the boss: I died to him, and when I back to him, this happened :
The attacking part of the boss separated from the boss' body; I was still able to attack its body and kill it.
The second bug I encountered when taking the elevator back up; when it transitioned to the next level I was presented with just the elevator shaft, and I couldn't see my guy, but I could hear my guy moving around. I was soft-locked at that point. This happened several times until it didn't, and I could see my guy on the elevator going up to the next level like normal.
My last bug was when I returned to the beginning of the game after getting the double jump and gravity gun; I saved/healed at the first save spot and my game immediately crashed. When I went back into the game, I was at that save point like nothing was wrong.
I hope this helps. Good luck with your game!