Robin Williams Secret Comedy Show in Hollywood

5/3/2012 8:47 AM PDT

Cool thing happened in Hollywood last night ... when Robin Williams dropped by a small comedy club on Sunset Blvd. unannounced ... and asked if he could perform.

It all went down at a place called Meltdown Comics -- which hosts a small comedy event every Wednesday night. We're told Robin walked in ... grabbed the mic ... and busted out a set for roughly an hour.  Sources at the show tell us ... Robin KILLED IT.

It's not unusual for big time comics to drop in to small venues and secretly test out new material before a tour ... or a special. We called Robin to find out if that was case with last night's impromptu set ... so far, no word back.