Another thing is that you won't get the "messenger asks you to let the crusade through" message if you use the spacebar to bypass the computer move. Thus if you see a crusade on your border and don't want it to enter (and don't care about being excommunicated), you *must* let the computer finish its turn so you get the message.

It's most frustrating, IMO, to look at your largest army, only to see it's been decimated by a passing crusade that never asked for permission to pass through (thus you didn't even know it was there).