Jon Gruden Claims He Was 'Tricked' Into Wearing Wrong Hat, 'I Apologize'

Jon Gruden's hat debacle on "Thursday Night Football" was the result of a practical joke played on him in the locker room -- so says the Raiders head coach.

Gruden caught hell on social media once fans realized he was wearing an "Oakland Raiders" hat on the sideline -- epic fail considering the Raiders just moved to Las Vegas!!

Social media clowned the 57-year-old for the error ... and the roar clearly grew so loud -- JG became aware of the cap problem in the 1st quarter and switched hats soon after.

"I apologize for not having the right hat on," Gruden told reporters. "Somebody played a pretty good trick on me."

So, who is this mysterious trickster? Gruden didn't name names -- but someone's ass is in the hot seat.

The thing is ... Gruden's headgear ended up being the LEAST of his worries against the Chargers -- 'cause the Raiders took an L they desperately couldn't afford to take.

Las Vegas' playoff hopes essentially rode on the game against LA ... but its defense struggled all night and the team lost in overtime, 30-27, to Justin Herbert's crew.

Gruden apologized for THAT as well ... telling reporters, "It's on me."