In 1997, the NFL introduced the "Emmitt Smith rule," prohibiting players from taking off their helmets on the field. The only exceptions are during timeouts, between quarters, and during player injuries.
Dez Bryant is in his fourth year in the league and did not know this rule. Bryant took off his helmet out of frustration in yesterday's Vikings vs. Cowboys game and was shocked to get called:
“He threw the flag and the whole time I’m thinking it’s a flag on him. Then I took my helmet off and I didn’t know that that was a penalty. I was kind of shocked and then I heard the ref say, ‘He took his helmet off.’ I know now not to take my helmet off. If you go back and look at the play, you’ll see I’m shocked. I’m confused.”