In the end of the film, when Aileen is sitting under the overpass along the interstate, you can see a late model Lynx bus, part of Central Florida's public transit system. Lynx didn't exist in the '80s.
After Aileen kills the first john, she stands over his body with her underwear around her knees. The shot switches to the dead man on the ground, then back to Aileen, with her underwear and pants all the way up.
At the beginning of the movie Aileen sits under an overpass and in the next shot she checks her revolver, apparently still in the same location. At the end of the movie we see the same shot of Aileen under the overpass, but in the sequence just before that we saw how Aileen has dumped the revolver already.
After they crash the red Grand Am, one shot shows a branch from a shrub under the rear wheel. Just before they drive a small piece of wood is under the rear wheel. As they're running away, all of the shrub branches are under the rear bumper. Suddenly, the branches are upright.
After Selby returns from the gay bar, Aileen confronts her in the kitchen. Two loaves of bread are on the second shelf in the cabinet. In the next shot, with the camera behind Selby, there is nothing on the shelf. A purse is missing from the counter.
Aileen comes back to the motel and finds Selby cutting off her cast in the bathroom. A can of beer is on the dresser in front of the mirror, and the lamp on the table is on. When they leave the bathroom, the beer is gone and the light is off.
When Aileen sneaks into the self-storage place, the closeup shows no shadows at all on the wall the building. The long shot shows shadows on the wall. The sun appears very low on the horizon (around 6am) instead of around 10:00 am, when Selby went to church.
When Aileen drops the gun off the bridge, she is crouched next to a double bridge pole on her left. The next shot from behind shows her centered between two bridge supports, with a double bridge pole on her right.
The 80's style cordless telephone Selby is using at her aunt's home to speak to her father would only have worked with the antenna pulled all the way out, but the antenna is down, so it is impossible for her to be speaking to anyone.
The postscript says Aileen Wuornos was executed after 12 years on Florida's Death Row. She was arrested January 9, 1991, sentenced to death January 31, 1992, and executed October 9, 2002. Even if you start at the date she was arrested, it adds up to 11 years and 9 months.
A revolver is not restricted to only hold six rounds of ammunition. Most large frame .22 caliber revolvers (the type of revolver used in the film and by Aileen) hold more than six bullets, with nine rounds being the most common in .22 caliber revolvers. The ammunition is much smaller, thus the cylinder of the revolver can easily hold and fire more than six rounds before reloading.
When Aileen meets Selby in the bar, a girl in the background gets taller as she walks between them. A platform was placed under the floor so Christina Ricci didn't look so short next to Charlize Theron.