Continuity mistake: In Season 2 episode 7, "Pretty Much Dead Already," at the end where Rick shoots the walker Sophia, she falls to her right. In the next episode, "Nebraska," picking up immediately after the last, she is in an entirely different position, as if she fell forward.
Character mistake: While Hershel is sitting at the bar talking about what happened at the barn, he tells Rick what he was thinking about as he watched Rick run across the field, carrying Carl, who'd gotten shot. But it was Maggie who'd witnessed this through binoculars, and then yelled for Hershel.
Suggested correction: And he came outside to see Rick running across the field with Carl. No mistake.