It happened just before 12:30 a.m. Sunday at the Century Boulder Theater on the 1700 block of 29th Street.
Boulder police are taking this prank very seriously and say it's no laughing matter.
The 17-year-old wore a Joker mask, which immediately brought to mind accused theater shooter James Holmes.
Holmes dyed his hair and police say he called himself the joker at a midnight showing of the latest Batman movie on July 20.
Twelve people died and 58 were hurt.
Moviegoer Eve Rose says that kind of prank would put anyone on edge.
"I thought it was incredibly poor judgment," Rose said. "A trivialization of what happened in Aurora and I was very, very sad to hear that it happened in Boulder."
Police say the teen admitted he wore the mask to scare his friends and wanted to dress as the joker next Halloween.
Police arrested him on five counts of misdemeanor menacing.
CU-Boulder law Professor Aya Gruber says, under state law, prosecutors must prove the teen knew his actions would cause others to fear serious bodily injury.
Under Colorado statute 18-3-206: A person commits the crime of menacing if, by any threat or physical action, he or she knowingly places or attempts to place another person in fear of imminent serious bodily injury.
"That may be terrible behavior, but it's not necessarily criminal behavior," Gruber said. "Is wearing a Joker costume in the aftermath of the horrible tragedy the same as, for example, displaying a gun to somebody or threatening to kill them?"
Inciting panic in a public place is against the law.
"You could have caused a stampede in that theater. Maybe someone else could have brought out a weapon in self-defense," Gruber said.
Police say the teen was not armed, he did not make any threats, and nobody was hurt.
"If somebody cries 'fire' in a theater that can cause chaos. And I think he had the potential to cause a lot of chaos," Rose said.
Police took the teen into juvenile custody and are not releasing his name because he is under 18.
Police say he was taking some classes at CU-Boulder.
The Aurora theater shooting has inspired several suspected copycat incidents in the nearly four months since the attack.
Maine state troopers pulled over a man and found guns, ammunition, and news clippings about the theater shooting.
In Maryland, police arrested a man who called himself the Joker and threatened to shoot up his workplace.
And in Colorado, a man was placed on a mental health hold for a tweet that thanked the shooting suspect for "making this world a whole lot better."
Boulder Police are asking anyone who was in the theater to call detectives at 303-441-3330.
The case number is 12-15529.
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