Plot hole: In the scene where the limo driver gets shot, Deloris walks in just as the bullet is fired. She doesn't see what happened before, just the guy getting shot. However, when she is telling the police what happens, she tells them that Vince looks at Ernie and says 'Good night.' He did, but she wasn't there at that point.
Deliberate mistake: When the nuns sing for the first time in the church in perfect tone then break into the dance version of the song, the reverend mother acts like it's the first time she has heard it. In reality, she would have heard the rehearsals as the convent is very quiet and you can hear the choir rehearsing when she tells Deloris she will join the choir.
Suggested correction: She may have heard it, but never seen it. What startled her were the moves.
Suggested correction: The only genuine mistake is that Doloris wouldn't see Vince looking at Ernie or Joey but as the office door is glass, she would be able to hear them talking. Example being when Deloris leaves and is standing outside with the door closed she can hear Vince telling Willy and Joey to bring her back for a talk or, if she runs, to "take care of it".
Except that she never stood outside the door listening to the entire exchange. As soon as she got to Vince's door, she didn't stop to listen to the entire meeting she immediately opened the door, walked in and witnessed the murder and nothing else. Plus, about five seconds after Vince says, "Say, goodnight, Ernie", is when Delores arrives.
To add, the only way for her to witness the entire exchange between Vince and Ernie was if she was standing in the room the entire time which she wasn't.