Letters on a desk in the lawyer's office have 33 cent stamps showing the US Flag and a skyscraper, which were issued in 1999. The real Erin Brockovich did her work (and the movie is set) in the early 1990s, when the first class postage rate was 25 cents, and later 29 cents.
When Erin confronts George about his loud Harley, the motor clearly sports an oval air cleaner cover identifying it as a Harley Twin Cam which didn't come out until 1999-2000 (depending on the model) - several years after that scene would have taken place.
After Erin comes up from the PG&E well with the water sample, she is chased to her car by two PG&E employees. She gets in and frantically tries to make her getaway. Apparently the car doesn't start the first time and we hear her trying again. However, when we hear the starter cranking the second time, Erin's hand is already off the ignition switch and moving to the gearshift.
When Erin is challenged to recall plaintiff's phone numbers by the newly brought on partners, and she is giving details to show she knows the plaintiffs, she states that a certain plaintiff has 5 children, but when she is listing their names she only gives four.
When Erin and George are sorting out the various decks of playing cards, she asks him how many decks they have. George responds, "We have more than we-nuff", an obvious combination of "more than we need" and "more than enough".
In the accident, Erin's car is seen being hit broadside by the doctors car obviously running though a red light while she was driving straight across the intersection, but in the telling of the accident in court Erin says that he came around the corner, out of control, as she was "slowly pulling out" of her parking space. In the visual, the doctor is clearly the guilty party but in Erin's courtroom testimony it appears that Erin pulled out of her parking space unsafely into the path of an oncoming cornering car.
Immediately after their first meeting with a PG&E attorney, Masry and Brockovich return to his office where we see him loosen his tie. After we cut to Erin, we return to Masry, whose tie is back in place. Next time it's loose again.
When the kids are playing what appears to be the card game War with George after the "Erin finds the kids with George scene", as Erin is telling the kids to go to bed, the children get up, and the son says "Good night, Aaron", when referring to George. Aaron is the actor's name, Aaron Eckhart.
(at around 47 mins) When Ed Masry hands over a stack of paper to his assistant, to be faxed to the PG&E Company, you can clearly see at the bottom of the stack that there are pages of a movie script, when he hands them over and when the assistant arranges the papers in a neat stack on her desk, preparing them to be faxed.