21. The USFL
The USFL was a thorn in the side of the NFL for three years. Conceived by David Dixon, he saw a market for football in the spring and summer outside of the traditional NFL and college football seasons. The main reason why it was an embarrassment for the NFL was the lack of salary caps.

This meant that a significant number of established NFL stars departed for the new league. In the end, the USFL folded after they tried to compete directly with the NFL. They technically won a lawsuit but only won a single dollar. It was very messy for the NFL though, who didn’t want the competition.