The only even remotely plausible explanation I can come up with for Glenn's survival is a sudden, massive distraction that makes the zombies turn away at the last second. They have been shown many times abandoning a meal to investigate a new sound.

The problem with my own theory: if there was indeed such a distraction, surely Michonne and Co. would have heard it?

 

Glenn is alive and survived by being coated in dead guts as we've seen in other episodes. The Walkers probably get distracted by...something, and stagger off after fresh meat.

 

I'm a bit behind with the episodes (only watched episode 3 today), so I don't know, if Glenn is really dead or not, but to me, it looked that way. He was clearly screaming in pain. I hate to see him that way, he was a nice character and although