It was happy hour at the Rio Grande Mexican Restaurant in Boulder, but something was amiss in Margaritaville, and it wasn't a lost shaker of salt.

The Rio's signature margaritas were nowhere to be seen Wednesday, as the restaurant served out the last day of a liquor license suspension for serving alcohol to an underage police officer.

The city's Beverage Licensing Authority handed down the four-day suspension for the Rio in January after an employee failed to card an undercover officer after a University of Colorado football game Oct. 1.

The manager of the Rio, Rob Trenz, said he expects the suspension will cost the restaurant about $10,000 in lost revenue over the course of the four-day suspension.

"It's pretty devastating to us financially," he said. "Our liquor license is an important part of our business."

But Trenz stressed the punishment was fair and that there was no one to blame but the restaurant and the employee, who was fired after the incident.

"We knew the consequences of this, and now we have to bear the shame," he said.

Mishawn Cook, tax and licensing manager for the city of Boulder, said punishment for violations is determined by the Beverage Licensing Authority, which normally follows a state guideline that takes into account the number of previous offenses.

Cook said there is some flexibility in the punishments. In a recent case, Folsom Field East Centerplate -- a company that hosts events in two rooms at Folsom Field -- was also found in violation of its liquor license after the Oct. 1 football game. It petitioned and received the maximum $5,000 fine, but Cook said those cases are the exception, not the rule. Most violators, like the Rio, receive suspensions.

Cook said Boulder has one of the most stringent compliance tests in the state. Police check all of the 266 businesses with liquor licenses at least once a year in stings, and Cook said they normally have an 85 to 93 percent compliance rate.

The effects of the suspension were evident Wednesday afternoon among the Rio's empty tables during a not-so-happy hour.

Gordon Chapman drove from Erie to meet his wife -- who works near the restaurant -- for dinner and was disappointed when he found out he couldn't order a beer or a margarita.

"If I had known before, I wouldn't have come here," he said as he ate chips and waited for his wife. "Even when I did get here, I was tempted to call my wife and leave. But I think I can go one meal without a drink."

He added that in a college town such as Boulder, he was surprised more bars aren't caught serving underage drinkers.

"I think you could get a violation from every place, every night in a town like this," he said.

Trenz, the Rio's manager, said in addition to police stings, he employs a private company to conduct regular compliance tests on his employees.

"Carding is one of the simplest, most fundamental and base functions of the servers, and we think they should get it right every time," he said. "But people make mistakes. This time it was a good kid that made a dumb mistake."

Trenz said the violation was only the Rio's second in 22 years.

"You never want to have that sign on your door," he said, pointing to the posted notice of suspension.

Friends Gillian Owen and Virginia Graeber didn't even notice the sign on the door when they walked in Wednesday and had to find out about the suspension from the waiter. Graeber said she didn't know if they would have chosen the Rio if they had known.

"I mean, maybe, but the margaritas are definitely one of the big draws," she said.

Contact Camera Staff Writer Mitchell Byars at 303-473-1329 or byarsm@dailycamera.com.