1) The fact that I texture a brush in VHE, compile it, and then when I see it in game its a COMPLETELY DIFFERENT TEXTURE. Like this:

2) For some reason, some brushes dont show up in-game. I place a perfectly good brush that is a mirror of another brush, and that brush just refuses to show up. I dont freakin GET IT! It shows up in VHE, but not in-game, once again.