Factual error: After JD steals Louise's money, Thelma decides to rectify the fact that they're broke by robbing a convenience store in Oklahoma. She holds the clerk at gunpoint, asks for all the cash, and then requests he hand over a bottle of booze. The problem is, due to the liquor laws in Oklahoma, one can only buy 3.2 beer and wine coolers at grocery stores and convenience stores, not "hard" alcohol.
Deliberate mistake: In the scene where T& L are being pulled over for speeding, when they park, you see no protective rails on the side of the street they are on, but after they put the officer in the trunk and return to their car, on the passenger side, the rails are very visible.
Continuity mistake: At the western bar Louise is drinking a margarita. As the shot goes back and forth between Thelma & Louise, the contents of the drink magically decrease and increase again. The level of Thelma's coke rises as well, when Harlan leaves them.
Factual error: As the Highway Patrol officer approaches T&L we see that there is a carrier for 12 rounds of ammo (obviously for a revolver most likely a .38 Spl or .357 magnum) on his gun belt. When he surrenders his weapon we find it is a semi-automatic pistol. The cartridges on his belt would not be compatible with a semi-automatic pistol which he was carrying.
Revealing mistake: Thelma and Louise in one scene are driving at a very high speed with Brad Pitt in the backseat wearing a cowboy hat that never flies off, much less even moves with the wind.
Continuity mistake: When Thelma and Louise lock up the policeman in the trunk they shoot two air holes in the lid. Later the policeman sticks his finger through a hole to point out to the bicyclist where Louise had thrown the keys (surprisingly he could tell from the rather feeble sound what that sound was and where the keys had landed), and the rim of the hole shows that the bullet was shot from inside.
Visible crew/equipment: In the scene where Thelma & Louise pass the tanker for the last time, before they blow it up, there appears to be an image of a man visible twice near the bottom of the tanker, just before the tanker cab is shown.