There have been so many impossible scenarios from the very first episode, that this one seems mild by comparison.
Rick. In a coma. The day Shane leaves him and pushes the stretcher against the door, the flowers on his table are live. Soldiers are killing everyone in the hospital. The stretcher stays in that position until he comes out. So, it's safe to assume no one every came in to give him fluids or clean him up, etc. When he wakes up, the flowers are DEAD, not a little, a lot. So lets say a week minimum?? And he went without any care or hydration in that time. And he wakes up clean, dry, and able to stand a walk, doesn't go immediately for water, the list goes on with that one. Highly unlikely.
Remember when Darryl and Beth were on the run, and they went into the house with the mortuary in the basement? Darryl never should have made it out of that basement. Not a chance. No way. BUT, I'm SOOOO glad he did.
Herschel should never have survived that bite at the prison. It was at least a minute or two before they cut off his leg. At a normal/slow heart rate, your blood has pumped about 100X by then, much too late to intervene and stop the spread by cutting off a limb.
Glenn at the prison, requiring an ambu bag to breathe, yet within minutes of receiving an antibiotic, he's back in the fight for his life against walkers. Pretty sure it doesn't work like that in the real world.
I'm sure there are many, many more I can't think of. But, I'm still good with it. It's a show. It's exciting. I love the characters. I'll be totally bummed if/when one of my faves does die. And, I CAN'T wait for next week. Whoop, Whoop.