Towards the end of the movie,"Jeffery" is pursuing "Lori" down a hallway at the studio. She wants nothing to do with him. As he pleads with her to listen to him,he calls her Elisabeth, her real name (Elisabeth Shue).
When we first meet Mark he is auditioning for Betsy as a waiter with one line. Later on in the movie we see Mark deliver that one line during a scene with Celeste, Lori, and Jeffrey. However, at the time of the audition, Lori was not a part of the production at all. Mark auditioned for a scene that, at the time, should not have existed.
In the scene where David and Montana are conspiring on the "beach" set, David is wearing his glasses, but when the angle switches to an over-the-shoulder shot from behind him, his glasses are suddenly off.
When Monica Demonico enters the set from the side after Jeffery Anderson re-acquaints himself with Celeste, a hand is visible on the door just after she enters the room. Off to the side, the only person waiting to enter the set is Bolt, Celeste's husband.
A poster in Kevin Kline's dressing room lists him as playing "Willie Loman." The name should be spelled "Willy." The production wasn't that polished to begin with, so it wouldn't be surprising if no one proofread the poster.