At 11:11, two customers enter the door as Sam and Diane are standing at the bar, but when the scene cuts to Frasier, the door is suddenly closed. Seconds later, as the scene cuts back and forth between Frasier and Sam, you hear the familiar squeak of Cheers front door off-screen even though the door is supposedly closed.
When Carla enters the front door, someone in a trench coat is partially visible standing outside waiting to enter. The coat is similar to the one Dr. Ludlow is wearing minutes later, meaning the person standing there is likely actor James Karen.
At approximately 11:35 when Sam calls Diane over to the bar, a male patron in the background hits his head on a lamp rather hard when seating himself at the table just to the left of the entrance door. The actor can be seen reacting to the knock on the head and the lamp begins swinging.
When Carla and Diane are talking in Sam's office at the pool table, the shadow of a camera or a piece of lighting equipment is visible in the lower left corner, and behind Diane, to the right of the cigarette machine. The shadow moves across the base of the wall.
Sam mentions that Carla has been receiving flowers from her secret admirer "everyday for the last 2 weeks." Just a short time later, Frasier retorts that his mentor (Carla's admirer) has been dismissing him for 2 MONTHS. Sam is correct; Frasier is in error.