What happens in Hollywood when two big name stars diss another big name star? We might have the answer... when it comes to Robert Pattinson.
The unbelievable scene went down last night at Chateau Marmont ... where Biebs, Selena, Robert and Katy were all hanging after Katy's movie premiere.
When the posse made its exit, Bieber, Katy and Selena all piled into a limousine bus -- surrounded by heavy security.
Unfortunately for Pattinson -- who was hangin' with "Raising Hope" star Shannon Woodward -- the bodyguards left them out in the cold.
The good news is that the door eventually opened again and Robert was allowed to get on the bus.
Had it not, we're sure Twitter would've been blowing up!