It was really nice to discover this game. I played Karoshi as a teenager and really liked it.
Besides the little glitches with the portal mechanics, I have a few design comments:
- The occasional level name like "r to restart" should be a different color or maybe something like #0077FF (azure). Blue with a black outline is hard to read since blue looks innately dark.
- Level 8 and 9 are really nice puzzle levels, probably my favorite, and Level 16 is a fun joke level.
- Level 19, the one with the 'no music' sign, I thought you had to go and mute the game. I never discovered the idle animation.
- The sticky goo doesn't mechanically seem on-theme with Karoshi IMO. I think it's too see-through as effectively a death block, when the fantasy of Karoshi is using the one death block (spikes) as progress instead. I would have used teleport blocks to set back progress in the rocket levels instead, which could also have created more opportunities for puzzles in the standard puzzle platformer levels.
- It was rude that there was a passageway in Level 21 after the spike. I thought there'd be an easter egg for going that way instead :P