From EW:

It is well known that instead of starting with Rick talking into his walkie-talkie and then the group being pretty much already out of Atlanta, the original plan for THE WALKING DEAD‘s season 2 premiere was to pick up just seconds after the CDC was blown to bits in the season 1 finale. I was on set playing the part of a zombie for an intense sequence in which Shane’s car breaks down just a few blocks away from the explosion, leaving him isolated from the group and having to fight off and outrun an army of the undead before finally being rescued by the group in the RV. But what happened after that?

In this clip following Shane’s rescue, we see the survivors trying to seek shelter with some familiar faces from season 1— Guillermo and the Vatos. But when Rick and Co. arrive back at the nursing home where Glenn was once held hostage, well…the people they find there are not people at all, setting off a frenzied attack.