Zombies that are entirely sentient, that seem completely unaffected, zombies in the perfect guise of humans who lie in wait for sometimes years, one million Tyler Dirdens waiting for their host to call them in that phone booth.
Every subconscious thing you've learned, every Manswers episode you've ever watched, if you so much as looked up napalm's composition on wikipedia, read a book about martial arts, or read about sensitive points on the human body, the zombie will know, the zombie will remember, even if you don't. That zombie will be better than you at using any weapon you've ever used, they'll talk and manipulate humans better than you ever thought you had in you. That zombie will be a darkness within the pit of your stomach that you can't fight, a homicidal psychopath just underneath the folds of your concious brain, scratching, biting, struggling to get out, until one day you wake up and you're not the same person anymore. Anyone you know could be a zombie, and you would never know. Not until it's too late.
You'll alternate between personalities, you'll try to stop yourself, you'll scream and fight yourself, argue with thin air, and never, EVER be able to stop it. Because there's only one answer to that bite of yours, only one way to stop the infection before the cure's been invented. Be you a shambling undead or slowly going insane, the only answer is death.
Sorry for spoiling that movie, everyone.