It's worrying because they're unpredictable and won't necessarily do the appropriate counter move.

For instance, I'm a horrible chess player, and I even tell my opponents that, but yet, they often take a much longer time to play because my moves are so unpredictable it really throws them off their own game (also---I'm regarded as a smart fellow, and I wouldn't make obviously stupid moves, so when I do, my opponent wastes time trying to figure out what they're missing).