Porn Star Andy San Dimas Busted -- Sorry My Sexy Unicorn Dance Screwed Over a Cop

Posing with porn star Andy San Dimas -- while wearing a UNICORN HEAD -- has a Pittsburgh cop in huge trouble, but San Dimas claims the whole booty-shaking mess is a giant misunderstanding.

This all went down last weekend during a Pirates game at PNC Park, where Andy says she was doing her best to get on the jumbotron ... by shaking her ass in cut-off shorts, while wearing a unicorn head mask!

Awesome, right? Wrong ... because Andy says when an usher asked her to sit down, she tried to rile up the crowd by shouting back, "Oh, I'm sorry I was trying to have a good time at the f**king Pirates game!"

Not smart ... Andy was hauled off to the Pittsburgh PD station at the ballpark.

Andy says the cops were very friendly, and told her they had to boot her from the park -- but not before one officer asked to try on the mask.

Not smart again. That "unicorn cop" is now reportedly under investigation. It's unclear if it's the unicorn mask, the porn star -- or both -- that got him in trouble.