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!"