At the beginning of the barn scene, just after the walkers come out of the barn, Andrea can be seen running from the group to Shane twice.
When Maggie and Hershel are talking in the kitchen, Maggie mentions that she was 14 years old when Hershel married Annette, who was Beth's mother. Unless Beth was born several years before Hershel and Annette were married (which is unlikely, given Hershel's religious views), Maggie would have to be at least 14 or 15 years older than Beth. However, Maggie's age is established as 22 and Beth is only about 5 or 6 years younger.
When Shane starts destroying the boarding over the barn, the last thing remaining is a chain strung between the top of the two doors. Shane doesn't take off the chain, but in the next shot after he slaps the doors, they open without resistance, indicating that the chain has disappeared.
During the confrontation between Maggie and Glenn, Maggie's blouse alternates between being on and off her right shoulder.
When Rick and Hershel return to the farm with the caught walkers, Shane and the group are gathered in front of the house. T-Dog is just standing behind Shane over his left shoulder and T-Dog's right arm is folded behind his head. Right when he says "Oh shit," the camera changes and T-Dog has moved further away from Shane with both arms at his sides (with no time to change positions).
When Dale and Shane are in the woods, you can see on Shane's T-shirt a couple of prints of the gun barrel even before Dale pressed his rifle against Shane's chest. Thus revealing that this scene had been rehearsed several times beforehand.
When Lori is cutting carrots and Shane is walking up to talk to her look in the background behind Shane as he squats down there is a person walking towards the barn.