Lichen has a solid collision box. You can see this by walking over a lichen block with the F3 menu open and watching the Y level change, by walking along a wall up to a lichen block, which will stop you, and, as I discovered it, by trying to climb a scaffolding tower to the top when lichen is on top of it, which prevents you from climbing it all the way.

#33

Groovy. I'll make this issue just about the lichen then, I guess.