No matter how friendly you think you are with your boss, a recent incident reported by Arlington police shows why it can be dangerous to mix work and after-hours alcohol.

On Wednesday night, according to police, a painter and his supervisor were drinking beer at the home of the supervisor’s girlfriend in the 2000 block of South Langley Street. The men were friends, and the night was going well.

But around 11 p.m., police said, the boss started telling the worker that he was slacking off on the job. The worker became angry, police said, and after an argument he walked outside and smashed his boss’ car window with a beer bottle.

When the boss came outside, police said, the worker apologized and said he would pay for the damage. Later, however, he apparently told his boss he would not pay for the smashed window.

That’s when the boss got violent, according to police. He grabbed a souvenir wooden baseball bat and unleashed it on his employee, smacking his head repeatedly with the bat, police said.

“He took his aggression out with a bat to the guy’s head,” said Dustin Sternbeck, spokesman for Arlington County police.

The painter suffered cuts to his head, a swollen eye and a busted lip, police said, and was taken to George Washington Memorial Hospital to be treated for non-life-threatening injuries.

The supervisor, Carlos Guadalupe Lopez Lopez, 26, of Hyattsville, was arrested and charged with malicious wounding. He is being held without bond, police said.