What did he do that was incautious?
He got out of the car.
Our neighborhood watch never gets out of the car. They call the cops. The only way anyone in our neighborhood watch would get out of the car is to stop an ongoing crime. While Martin may have been acting suspicious, there was no indication of an actual crime going on. So, Zimmerman should have stayed in the car. Some people may disagree and say that Zimmerman was just being proactive, but I am sure that others will learn from this event.