As seen in:

Night of the Living Dead, Diary of the Dead

Why it's a sign:

Don't be fooled by their barely functioning nervous system and shambling gait, Zombies have a devastating sense of timing. And if there's one thing they appreciate more than brains, it's dramatic irony. So if you're walking through a graveyard with a woman who's scared of the undead, you should avoid putting on your most sarcastic spooky voice and saying "They're coming to get you, Barbara." For whatever reason, sarcasm sounds like a dinner bell to zombies.

She's not reacting to your devastating sarcastic/spooky voice.

Zombies are incredibly patient. You can go your whole life avoiding any zombie irony--never dressing up as a zombie on Halloween, skipping all the "dead people are dead and never coming back" parties your friends throw every Easter. No rush. Take as long as you want. Zombies aren't busy. They'll just be chilling out under a thin dusting of suspiciously loose soil. And the second you slip up and do something that would make a zombie attack the least bit ironic, like say, starring in a zombie movie, they will be on your ass with a quickness.

That's because the deeper people get into a zombie crisis, the more everyone starts acting like total assholes. So skipping two weeks of consciousness doesn't make for the smoothest of transitions. Your new friends got to ease their way into the apocalypse, whereas you just rolled out of a warm bed, directly into a frigid pool of cannibalism and back-sass.

NOTE: This of course assumes that you survive the two weeks in your hospital bed serving as a man shaped hot plate for your brains. For whatever reason, this is generally a safe assumption.

As not heard in:

Every single zombie movie ever, and mentioned in Shaun of the Dead.

Why it's a sign:

Let me set up a scenario for you. You're coming home from work one evening. You pull into your driveway and pop out of your car to see your now-dead friends walking up the street, with the intention to eat you. What do you call them? If you said "them" or "things," then, well, that isn't a far-fetched scenario so much as it's a glimpse of your future. Make no mistake: With the exception of the aforementioned metatextual zombie movie, nobody in a zombie movie has ever seen one. If this page is in your browser history, it's probably time to be scared.