I kinda hate it when this happens. But I also kinda like it when this happens. Neal Whitman beats me to the punch.
I had been composing (mentally) a similar post on the somewhat anomalous tense and aspect of Joe Wilson's ejaculation: "You lie!"1 With most other verbs you'd expect the progressive aspect marker auxiliary are and a progressive participle verb with -ing suffix in this context, where the intention is probably something like You are doing this right now.
Wilson could have been implying that this is something Obama does all the time. Much like You drive so slowly! This of course is usually said when the behaviour is currently … being … done. And the utterer is fed up.
But for some reason lie lends itself to this form with the notionally simpler sense of just you are doing this right now or even you have just done this. Other accusatory phrases of disbelief seem to work this way: you kid, you jest and perhaps you don't say (as mentioned by a commenter over at Neal's place.) Stop by over there and share your thoughts.
1. But had I been left to myself I likely would have come up with a bloated and meandering post. Nothing like Neal's sharp work.