That sinking feeling

It's worth pointing out that the only thing preventing the quirk we saw last time from occurring with completely solid level blocks is wall collision: as you may recall, whether a block is solid from the top or from other directions is determined by two separate flags, and the associated behavior is also defined separately. As such, just like top-only blocks, if you're in the air and have any upward momentum, you will not register any collision with the top of fully solid blocks.

Ordinarily, this isn't an issue, because the player will always be falling when they touch the ground. The problem occurs when a level event sets the solid background flag and begins scrolling the background plane upward. In this scenario, it is possible for the floor to rise faster than the player, and since the player is moving upward, no collision is registered.

This can happen during the rising sand sequences in Sandopolis Zone, as well as directly after the Carnival Night Zone act 1 boss, since during this sequence, the foreground plane contains the boss' breakable blocks, where as the entirety of the shaft, including the floor below, is part of the rising background plane.

It doesn't happen in Marble Garden Zone because the player gets pushed up along with the background plane, even in mid-air, and it also doesn't happen at the very end of Flying Battery Zone because there's an invisible solid object rising along with the long floor beam.