Sometimes it appears as if there is a brick in my cart, but only if I look at my inventory in the cart. it's not in my actual building inventory if I press 1. If i double-click a brick it just overwrites the 'fake' brick. I get this quite often and I believe it has something to do with inventory favorites (I use hotkeys for them by the way). It's pretty confusing when you're clicking 'done' several times to get a brick that isn't there.