9. The Group Never Had A Rendezvous Point After The Prison Siege
To give credit to Rick and the group, they probably didn't expect The Governor to ever return to the prison, let alone with a small militia that even included a tank. All things considered, the group did pretty well with the whole ordeal, Hershel being the only main cast member to die in the battle.
Unfortunately however, the prison fell and everyone was forced to hit the road.
In the chaos of the battle there wasn't exactly much time for everyone to group up and leave together, meaning everyone split off into small groups. But you have to wonder, why didn't the group have a rendezvous point? Sure they might not have expected an attack from a tank, but it's incredibly naive of the group to think that the prison would never fall.
Having a place to meet up in a worst case scenario seems like something you'd figure out pretty early on.
How about Hershel's farm? Or the supermarket from the fourth season's premiere? Anywhere the group can just stay for a few days at most to re-group before moving on is all they needed. Instead they had no back up plan in place, and all ended up walking into the Terminus trap. Nice one.