A pitfall is a trap or difficulty you didn't see. A possible pitfall for high school seniors is not working hard after they know they've already been accepted into college.
In 1300, pitfall referred to an animal trap, such as branches laid across a deep hole offering no escape. Since the 1580s, pitfall has come to describe any hidden or unexpected difficulty, and its meaning has expanded to apply to humans. You might encounter a pitfall when trying to use a brand new computer program that's unexpectedly precise. It can even be a habit you don't want to address, like the pitfalls of snacking in front of the television every night.