Exclusive to the console versions of the game, there is a very small, randomly-generated chance of a Zombie having a nut allergy, meaning it can die when it tries to eat a Wall-nut or a Tall-nut. When this happens, a rare death animation is triggered where the Zombie suffers a severe allergic reaction until it collapses and dies.
Contributed by MehDeletingLater
By hacking the game, it is possible to access ten unused mini-games along with four variations on survival endless. All mini-games are fully functional, with the exception of Ice Level.
Contributed by vinoperson123
In the "Achievement" menu, continuing to scroll down the page, past the list of achievements, will reveal a number of characters and items referencing previous Popcap games.

At the very bottom of the page are Zombies dressed in Chinese attire situated in front of a pagoda, implying it is the other side of the Earth.
Contributed by ILoveMaMum
The lawn string text codes in the PC version of the game contain names of three enemies that are never seen in the game: Dog Zombies, Walking Dog Zombies, and Propeller Zombies. The reason why they're not in the game is currently unknown.
Contributed by frankfrost2
The Minigame 'Zombiquarium' is a direct parody of another PopCap game called Insaniquarium.
Contributed by ThisGuyInTheSuit
There is a zombie called a Dancing Zombie that looks like an undead Michael Jackson from Thriller (that even does the moonwalk). He even summons back-up dancers that look like they are from 80's. His description says "Any resemblance between Dancing Zombie and persons living or dead is purely coincidental." After the death of Michael Jackson, the zombie was redesigned to look more like a disco dancer, and the back up dancer were redesigned to look like they were from the same time.
Contributed by gamemaster1991
Plants vs. Zombies was at one point going to be called "Lawn of the Dead", a pun on George A. Romero's Dawn of the Dead. The name was changed for legal reasons.
Contributed by DidYouKnowGaming