Sex - Oh yes! Sex is good, but in a horror movie it is bad. If you starting humping a pretty girl, then she will probably grow big teeth and eat you, or if she is innocent a big machete-wielding maniac will come up behind you and show you the true meaning of the word 'pentration'.
Sugar - If someone in your group is consuming a lot of sugar then they are being controlled by some sort of insectoid parasite. Ask them to switch to sweeteners and if they try to tear your face off with their toes, you can be pretty sure that they are beyond saving.
Slippery - If you are walking along in the dark and suddenly step in something slippy, you have just steeped in the bloody remains of your best friend. Best bet is to just keep on walkin', pardner.
Slug/Snake/Scorpion - Anything in a horror film that spends its time crawling around on the floor needs to be stepped on immedietly. To avoid these things, you shold probably stay out of the jungle, unless you are looking for a rare flower that is the only hope of curing your dying wife.
Satan - The Big Bad. Currently the head of Wal-Mart, Satan is a big believer in high-volume, low prices. He is also a grade 7 pianist and a fond lover of Opera. In a horror film he will try and rip your face of and paint the walls with your blood.
Sand - Beaches are great, but if you ever find sand coming from 'above you' then you have been buried alive. Or you are in a sandstorm. Both are pretty bad so I'd suggest a nice lay down while you think about your life and how to save it.
Stupid - In a horror movie, stupid = dead. Don't be stupid; take that college course. The more you know, the better you will do, but don't learn too much. The geek always dies in a horror movie.
Skin - In a horror movie, people may be hiding things beneath their skin. To counter this, everyone in your group should slice their arms with a blade to show that they bleed real blood and that they are human.
Slime - Stay away from Slime!
Slimer - Stay away from Slimer! The little green bastard will get gunk all over you.
Stream - A gentle stream can be relaxing and a good form of refreshment. However, if you discover that you have passed the same stream SIX FREAKIN TIMES, then you are in the Blair Witch Project and no one can help you.
Stink - If something in that old abanondoned house stinks then it is either a dead body or a crazy cat-lady. Just get out of there and go to Taco Bell instead.