If you increase your health with the heart canisters in one world and go to another, the health increase carries over. I'm thinking that's not intended.

Definitely not intended... Hilarious, but not intended.
(edit) I think this will get fixed when I implement the player-based difficult system and rework data saving, so it might be a couple days.

Can't reproduce in 0.2.0, so I'm guessing this is fixed by the new save system.