2. Not Pursuing Andrew
Really, you should always finish jobs once you start them. Whether that's filing your taxes, building a house or offing obvious traitors, you should always follow through with what you're doing. So when Rick deals with the two murder-y prisoners in the opening throes of season three, you'd think he would've committed to it whole-heartedly. After all, Tomas and Andrew were clearly both out for the group's blood, so it's a necessity that you have to deal with both problems. Being fair to Rick, he deals with Tomas pretty damn equivocally, by straight-up killing him. However, he decides not to track Andrew, and to me, that's just plain stupid. Here you have an obviously violent man who knows the prison lay-out, and seeing as Rick's deprived him of his stable existence, has a pretty obvious beef with you. Yet instead of tracking the guy down it's not as if there's a tracker in the group, right? he leaves the thing unaddressed, and inadvertently causes the death of his wife. Really, if he'd have just grabbed Daryl and hunted the bloke down, we'd have been saved crazy Rick and we could've seen a proper redemptive arc for Lori. As it was, it all went to hell in a handcart.