When Norma is holding the screenplay being written by Joe and Betty, their names are joined by the word 'and'. This would mean that Joe (who is listed first) had written the entire screenplay, sold it to a studio, only to have Betty revise/rewrite the script. Since the two characters were working together, their names should have been joined with an ampersand ('&').
When Max is telling Joe about directing Madam's first pictures, there is a bad dub of the word "sixteen". After the Roscoe 'Fatty' Arbuckle trial and the subsequent establishment of the Hays Office to enforce the new Production Code, the producers were concerned that the original age of 14 would be considered child porn and had the line changed in post.
Wallace Reid died in 1923, three years before Paramount's Marathon Street studio opened, thus he could not have had a bungalow on wheels on the lot as Max had pointed out (unless of course, he was pointing towards Sunset and Vine where Paramount had its lot at the time of Reid's death).
When Joe is sneaking out to work at the studio with Betty, he pulled the Isotta Fraschini out of the garage forward. When he came back, he also pulled it in the garage forward leaving proof that he was taking the car.
During the bridge game, the opening lead has been made out of turn. Norma declared the 3 No Trump contract, so the opening lead should have been made by the player to her left. However, as her partner (the dummy) is laying his cards on the table, the card led is clearly in front of the player to Norma's right.
When Joe is initially examining the scripts, Norma is shown with a very short cigarette butt. The camera pans to Joe and then back to Norma, whose cigarette is now (seconds later) significantly longer.
When Norma and Joe are being driven to Paramount, Norma raises her left hand to the left side of her face as Max adjusts the rear view mirror. After the cut, Norma's reflection in the mirror shows her hand immediately relocated to the right side of her face.
While Betty stares at Joe and Joe asks "What's matter?" we can't see a pen in his hand (at least the pen is not in a perpendicular position) and we can't see a cigarette box in his shirt pocket. But next transition scene there is a pen in his hand (in a perpendicular position) and there is a cigarette box in his pocket.
The morning Joe Gillis wakes up after his first night in Norma Desmond's house, he sees all his belongings in his room over the garage. Angry, he puts on his jacket over his shirt and leaves the room. In the next shot, when he is walking down the stairs, his shirt is inside his trousers.
When Norma Desmond drives through the Paramount gate, Jonesy, the guard who let her in, dials his phone and speaks into the phone, asking for "stage 18" before the phone's dial has even returned to zero.