Chris Brown SNEAKS INTO JAIL!!!

Chris Brown pulled a fast one Monday ... turning himself in at an L.A. jail for his hit-and-run case -- a day head of schedule.
 ... Brown showed up at the Van Nuys jail at around 2 PM.  We're told his lawyer, Mark Geragos, engineered it all and the purpose was to make it happen without any photogs present.

Brown was booked and released on his own recognizance -- no bail.  We're told the whole thing took around 45 minutes.

Now here's what's interesting.  Just a few hours earlier, there was a gaggle of photogs at the courthouse which is connected to the jail, because 50 Cent made a court appearance.  When the cameras were gone, Chris appeared.

Sneaky, Geragos.