I hate it when people don't realize their turn signal is still on

Have you ever found yourself behind a driver who's got their turn signal on and who apparently forgot to turn it off?  Are they going to turn?  Did they really forget?  Do you take a chance and pass them?

Or worse yet, you're going down a three-lane road, and the driver signals from the right hand lane, crosses over to the middle lane, and then keeps going straight with the left-turn signal on...  what next?