When Dr. Reese and Gabriel are viewing the clones, Dr Reese's lab coat is dirty at the top right in the beginning but the same lab coat is clean in the same spot in the later shot. Then it is dirty again.
All gunfire in the film is obvious CGI, as guns do not cycle and no empty shell cases are ejected from them when they fire.
After Gabriel escapes the torture room he shoots Doctor Reese in the right thigh while trying to extract information, yet pretty much as soon as he stands back up, and with no treatment at all, the Doc is walking perfectly normally without a limp for the next few minutes.
When Gabriel is leaving after his second turn to Dr Reese he comes to a railroad-crossing. At first you can see him driving his truck and stopping for the train in the left lane with a second truck to his right. In the next second in the overview you can see his truck in the right lane with a sedan to his left and when he drives off you can see through his side-window that he is in the left lane again.
The menu in the diner (seen while the sheriff is talking after he kills the fly) is labeled "Wallace Barbeque" not Gabes Diner as the sign outside says. You can see how they tried to hide it by turning the menus inside out.
During the gunfight battle at the "Doctor's Office", at about the 1:01:30 mark, the cameraman's arm is visible on the right side of the screen for a fraction of a second just after Gabriel exits the building.