Does opening the hard door delete your save? I noticed the elevator opened up where I was before (level 15), kept closing the game through esc because I couldn't find another way back to the menu and then went to see the hard door, but when I next loaded up after that the elevator was back at 000.
Trying to jump off of slippery floors angers me. The pop-up spikes sometimes can kill you if they pop up from a previous death and haven't had time to go back yet and I think there's some very specific time on somewhere around level 6 I noticed one of the spikes kills you without playing the death sound.
It's one of those games where it wouldn't be fun without good level design, and I think the levels are well designed, so that's good. My favorite level is probably the one which introduces falling from the bottom to the top of the map that you jump across twice in an X pattern.