Most of the stupidity I've seen involves driving.
I've seen people come to a grinding halt on the freeway because they missed their exit. They check their mirrors, look around, and sit there trying to figure out how to get to the exit they just missed. For some reason, it didn't occur to them that the easiest/safest solution is to take the next exit, and back track to where they were supposed to go.
I don't know if it's an LA thing or not. But there were quite a few occasions where I'm driving in traffic and the lead car just comes to a stop at the intersection.. when the light is green. The driver just sits there and waits until the light turns yellow, and then red. As I make my way up the line of cars, I notice the person is either talking or texting on their cell phone. Definitely a WTF moment.