Items hit by a projectile seem to continue moving forever. They just do so at a very low speed. It's as if there was no friction for items moving very very slowly.

This is especially easy to notice on maps with lava. You shoot some powerup, and half a minute later it slides into lava for no apparent reason.

Surprise, it affects even player character.

To reproduce:
Jump left or right (easier to notice) as far as possible. Land in such a way that you slide slightly (about 1 voxel) as you hit the ground. Then take your hands away from the ground and wait.

After waiting for 20 seconds or so (no actual measurement) you can see that your character moves forward about 1 voxel in that direction. If you aren't sure, you can confirm anyway due to the fact that your character switches frames of animation. So if you play a room full of pillars over lava, make sure to actually pause the game when going to a grocery store, for example. Even if no monsters are around.