The time on clock can be seen to be wrong multiple times throughout the series in the Mars Investigation office.

first season cant remember the exact episode but when Keith and Veronica are eating in the diner, when shes about to leave she put on her hat then they turn the camera to keith and when they bring it back to veronica she puts on her hat again.

In the episode "You Think You Know Somebody":
-Veronica logs into the P.I. website using her own name, but she doesn't have a P.I. license yet.
-When Veronica drops the boys off and remains in the car to talk with Troy, she has her sweatshirt draped across her shoulders. During Troy's close-ups, however, you can see that she is wearing only her t-shirt.
-When Veronica is yelling at Troy about his past he closes the locker at one point and the scene cuts to her right after and she says "Don't even!" but the locker is still open, scene cuts back to Troy and the locker is closed again, in fact, you can even hear the locker slam on the part where we can see it open when it cuts to Veronica.

"Weevils Wobble But They Don't Go Down"
When Veronica approaches Jaleesa Jones in the food court, Jaleesa's standing at the counter reading a book, which she's holding with both hands. When she answers Veronica's question, she puts the book down and puts her left hand on the cash register. The camera angle changes to show Veronica, and we can see that Jaleesa is still holding the book, which she then puts down for a second time

In "Like A Virgin" Meg is seen doing a live brodcast of the school's news that morning. In "Betty & Veronica" she tells Veronica that they are recorded the previous night. :p