Continuity mistake: In the diner, when the camera shoots the twins from the other side of the table, a sugar bottle can be seen. However, when it shoots from Bob's side of the table a bottle of hot sauce can be seen (it should be seen from the other angle as well, but is absent).
Continuity mistake: When they are getting on the bus after being turned down by some agents there is a guy with a red shirt getting on the bus. The scene cuts and shows the guy with the red shirt smoking but not on the bus. The other people in the shots are mixed up, too (the only one that stays the same is the girl Walt hits on).
Audio problem: At the start of the film we see the twins flipping burgers in their restaurant. We see that while one is throwing the burgers the other is catching them in the buns, and then dusting them with a metal jar of something. After every burger he bangs the jar on the table a few times. One one occasion, he actually bangs the jar on a burger bun, yet the sound effect is still played.
Visible crew/equipment: When Bob and Walt are being wheeled out of the hospital after the separation, just when the doors are opening in the reflection of the blue on the left, you can see the entire camera crew move along as well as the camera. When the camera disappears, another crew member can be seen running back.
Continuity mistake: We see that the join between Bob and Walt is very flexible, and can be twisted and contorted easily. Why is it then that when the brothers are lying in bed and Bob turns on his side to sleep, does Walt move with him. They should in fact be facing away from each other.
Factual error: First of all, Walt & Bob couldn't be conjoined twins. Conjoined twins have to be identical. While not all identical twins look 100% alike, they still have some similarities. Bob & Walt look nothing alike. Second of all, conjoined twins attached at the liver are the easiest to separate. The liver regenerates, so even if one was left with less, it would still grow back to the correct size. They could've been easily separated.
Plot hole: Bob and Walt supposedly share a liver, which apparently Bob has three quarters of. This would mean that the liver is in the join between them. All of the rolling around, twisting and banging of this section between them would severely damage their liver, and probably kill them.