In the level Timberdale, in the water tower area, before entering the water tower, have a triple jump ready. Jump on the level geometry to the right of the door, then do an extended high jump onto the top of the door. Then triple jump on top of the tower. Since the heart hitbox is still in place before you enter the tower, you might be able to get this heart this way, but the only thing I found was a softlock that clipped you through the level and into deep space under the floating island of timberdale.

I reproduced this a few weeks ago. Cool find, but I haven't been able to find any use for it.

It might be interesting to see what happens if you manage to hit the loading zone for the tower from the out-of-bounds side.