Insanely niche and specific bug:
If you use the space bar and arrow keys, it is physically impossible to jump, hold the key down and move left at the same time.
It does not matter which direction you're facing. You can go to the right perfectly fine.
If you change only one of the inputs to z or x it works, it is simply a combination of space, up arrow, and left arrow.
I learned this the hard way on the tiered mouse level, I couldn't get the key back no matter how hard I tried and it made me think that jumping with the key down to the right wouldn't work either which, on the rare occasions that I was able to retrieve my key, made me try to solve the level without lowering the key, which made me lose it even more.