What goes up won't come down

Some time ago, I mentioned how level blocks have two different collision flags: one makes them solid from the top, the other makes them solid from everywhere else, and they can be combined in order to make a block completely solid.

Blocks which are only solid from the top have some interesting properties, and I think it's worth pointing them out. First off, since you can only interact with the top surface, their undersides are often quite rough.

This generally has no negative effect, but when there are two such blocks stacked on top of one another, occasionally your character may end up standing on the lower one instead of the top one as intended.

More important is how these blocks work, though. They're supposed to allow you through if you jump from underneath them, but then catch you when you're falling back down, right? So the way the developers programmed them is that if you're in the air, and you have any upward momentum, then the blocks are completely intangible.

This leads to a pretty obscure mechanic, which is nonetheless acknowledged by the game's level design. If you have a sloped surface made out of top-only blocks, you can pass through it horizontally so long as you're in the air and moving upward. For instance, early on in Hydrocity Zone 2, as seen above, if you chain a spindash into a long jump, the speed will carry you right through the water slide and onto a ledge with some hidden goodies below.

Most places where this is applicable are noticeably barren, however, save for the occasional spring to get you back out. Notably, in one of the dumbest layout failures in the game, this trick allows quick and easy access to Hydrocity Zone 2's sewers, as demonstrated by the legendary GoldS in his glitches and oversights series.