1. When you think there's glass but there's not.
You know what's the worst? Running into glass doors. You know what's the best? Watching other people do it.
Of course, the funny factor goes down significantly when those in the latter party get hurt in the process, but most of the time, run-ins with glass doors do more harm to people's pride than they do to their bodies.
To honor this staple of human stupidity, let's take a look at some of the best gifs that the Internet has to offer.