The crate bugging you may be referring to is when you're standing on a Deco, which the server doesn't want to consider as solid, so your player model falls into it and you basically vanish. But on your screen, you're still on top of the crate since your client knows that it is solid. This issue has not been fixed.
Hillbugging seems to be caused by Phys_Walking or some other kind of physics prediction which wants to slide the player down into the ground. If you stand still and don't move, in about a minute, your solider will have lost his feet as if he was standing in quicksand. On a hill, things get pretty bad since the soldier can potentially slide down a mountain, which results in people wasting 5 clips on a player who can't die, and get shot out of nowhere by the real player who is invisible. This issue was addressed, but never fixed.
Leanbugging and Invisbugging are pretty much the same issue Yurch was talking about. UT uses the centerpoint of a mesh to calculate whether or not you have line of sight to it. If you were behind an open window that covered enough of your waistline, poof! You disappeared. Same goes for other meshes like Lamp posts and cars and tanks. If you look away from center point, they are no longer relevant and *poof* they vanish. This issue was fixed for players, but many mappers do not set their decos to AlwaysRelevant, which causes that strange effect where signs, crates, trees and bushes, etc, to suddenly appear or disappear.
What were we talking about? Oh yes, grenades that fire 300 shrapnel. Interesting if you want to run a physics simulation. Terrible if you want to play an online shooter game.