6. Watching Trailers
I know a chap who takes his iPad to the cinema and, when the trailers come on, he puts his earphones on and watches Bruce Springsteen rock videos at high volume so he doesnt ruin any films he hasnt seen yet. He looks a tiny bit mental, but hes clearly a genius, aside from the whole "suspected piracy" thing. Trailers spoiling films is by no means a new thing. In the 30s and 40s trailers were effectively just shots from the film with massive captions like SEE OUR HERO FIGHT AND DEFEAT THE EVIL ROBOT TIGER AFTER ABOUT 53 MINUTES or WATCH AS HER HEART IS MELTED BY HIS ROGUISH CHARMS AND BANJO PLAYING SKILLS. Nowadays its effectively the first third of the film on fast forward, three of the key action sequences, and an out of context line from somewhere else in the movie that sounds cool. As a result you spend most of the film going yep, remember that from the trailer - such as, specifically the bit where Bruce Willis uses a car to take out a helicopter.
Tip to avoid spoilers: Some combination of a blindfold, earplugs, and memory wipe system will work a treat.