Was there some break dancing going on in the wee hours?!

Rihanna hit up Roxbury nightclub in Hollywood last night, and the "Diamonds" singer looked picture perfect when she arrived in a pink crop top sweater and tight black pants, but on the way out we noticed dirt all over her knees!

What could have happened?

According to club insiders, RiRi was simply the unfortunate victim of an unwanted alcohol bath! "There was a drink that accidentally spilled on her and so her people dried it with napkins and the napkin residue left those stains," a source dished to HollywoodLife.

At least her dirty clothing wasn't the result of a vicious cat fight with nemesis Karrueche Tran!