One thing that gets us all into trouble on the roads is cutting in to a gap that's too small for us. The reason I think it's particularly dangerous is that the drivers closing the gap slow down to avoid killing you. This may lead you to believe you judged the gap just right, and pull into similar gaps in the future where drivers are not being as alert or courteous. You'll never know they slowed down. My own response, which is even more dangerous, has been to maintain speed a little bit longer to give the clear impression that the gap you pulled into was too small. It's a bad idea, and I don't do that any more.
Mokalus of Borg
PS - I'm sure there are plenty of dangerous, stupid things done on the roads daily.
PPS - And not all of them in cars.