How could Britney's favorite thing on earth treat her so wrong? Nooooo ... I don't mean Jason, I mean her beloved Frappuccino!

When Brit went to Starbucks yesterday in hot pants, a see-through wife-beater, tube socks, and a teased-up mane, she got a suuuper bad case of brain freeze after a quick suck on her straw.

But we just think the girl was lookin' so cute -- she was playing with us with that outfit, right? Brit's still got her old sense of humor ... sometimes!