Continuity mistake: After The Governor shows up at the prison with his arsenal, Hershel and Michonne are taken out of the vehicles and placed kneeling on the grass side by side, about 1 arm's length apart from each other. But when Rick makes his way down to the fence to talk with The Governor, Hershel and Michonne are now about 4 arms' lengths away from each other. This goes back and forth for a few shots.
Continuity mistake: When the Governor and his group show up and bomb one of the guard towers to get Rick and his group's attention, when Rick first runs outside the guard tower on the far left is on fire, but when Rick walks down to the fence to talk to the Governor the guard tower in front by the front gate is the one on fire. And it switches back and forth between the two. (00:16:05 - 00:19:15)
Factual error: When Lilly comes across the Governor's body, she points her 1911 at him. In one shot, we see her from behind with the hammer forward on her 1911. In the next shot she immediately kills the Governor by shooting him. A 1911 will not fire unless the hammer is back. (00:49:00 - 00:50:00)