Usually, when you're riding a bike and somebody in a car does something mystifying on the road nearby, you don't get a chance to follow up and ask what the heck just happened. But I had an exception the other day, and it's pretty interesting.
So, I was arriving at a co-worker's house for a party, and as I approached the driveway I signaled a right turn (using the straight-right-arm signal). In the oncoming lane a car was approaching, signaling left to turn into the same driveway. I looked the guy in the eye. Everything seemed cool. Since A) I was there first, and B) left-turning traffic is supposed to yield, I went ahead and started my turn...and the guy in the car started his. We both slammed on the brakes. No harm done, but a little disconcerting anyway, since I had clearly signaled what I was about to do, right?
Once we got to the house, I asked the guy (in a friendly way) what happened. He said: "I thought you were telling me to go ahead and turn."
So, there you have it: when you signal, don't assume that people understand that you're signaling your own intention. Live and learn.