I'm new enough to bike commuting that I sometimes still make the mistake of assuming that I know what a driver is thinking or how s/he will react in a given situation. It happened twice today. The first time I was approaching an intersection with a light. There's a sign that clearly marks it a "no turn on red" light. I could hear a car coming up behind me, but we were pretty close to the intersection, and since it's a no turn light, I figured there was no risk of a right hook. Wrong. The driver whipped around me, slammed on the brakes when she got to the light, and then quickly turned right. Fortunately, I was already slowing down for the light, so I stopped in plenty of time to avoid hitting her.
The second time was with a driver who really confused me. He was ahead of me in the lane and wanted to turn right. No problem, right? He should just turn. I was behind him in traffic. Instead he stopped, turned around in his seat, and waved for me to go ahead of him on the right before he turned. Weird.
Drivers can be unpredictable. Be safe.