Amanda Bynes has been acting increasingly unhinged for a while now, and being arrested late last week seems to have pushed her off some imaginary ledge.

Not only is she claiming the bong she tossed out a window was really just a vase, she's threatening to sue pretty much everyone -- and now, for reasons no one can quite explain, she's lashed out at Rihanna with some particularly appalling words.

In a series of quickly deleted tweets on Sunday (May 26), she told RiRi: "@rihanna Chris Brown beat you because you're not pretty enough" and "@rihanna no one wants to be your lover so you call everyone and their mother that I almost named my new dog Rihanna."

Uhhh. Wut?

To her credit, Rihanna had a damned zingy rebuttal:

Amanda responded in yet another tweet that she didn't leave posted for long: "@rihanna unlike ur fugly faced self I don't do drugs! U need the intervention dog! I met ur ugly face in person! U aren't pretty u know it!"

Damn. And to think it's usually Rihanna whose judgment we question.