Shaun not getting butchered between his house and the shop after the outbreak. His hungover state meant he more or less blended in.
The reason why Dianne was able to survive on the meat of David's leg according to the plot hole comic on the DVD. He was torn to pieces and there was no chance that the cause of zombification could've infected the leg.
The joke involving the winchester rifle in the titular bar is instantly more clever once you remember the rule of Chekhov's Gun "If a rifle is mentioned in the first act, then in the second or third act, it absolutely must go off!"
Why does zombie!Pete suddenly show up naked at the Winchester at the climax of the movie? Earlier, when Shaun first discovers that Pete's been zombified, he tries to pretend that everything's normal and casually invites him to join them at the pub when he's feeling better. Even as a zombie, Pete remembered the invitation, and he remembered that Shaun and Ed would be at the Winchester.
Because people are sleeping in the same bed with the undead and playing video games with them, there is more of a chance someone will be bitten and restart the Zombie Apocalypse.