Zombies
In-game example: Resident Evil. The walking dead. The world being taken over by malevolent, putrid corpses.
Robots
In-game example: Terminator 3: War of the Machines. Autonomous humanoid made out of metal and inevitably programmed to kill.
One of the biggest in-game fallacies. Our PCs show them killing, shooting, murdering and doing all sorts of human-like things, whereas the reality of robots is a whole lot different. They do exist, but we’ve barely managed to get them to stand upright. They’re either infants, toddling around and looking for help from their owner, or they’re automatons, mindlessly screwing bolts and spraying cars. But there are some impressive advances in the field, such as the self-correcting robot Big Dog. Only those with exoskeletons will survive!
Gravity gun
In-game example: Half-Life 2 . A chunky gun in Half-Life 2 and Doom 3: Resurrection of Evil that drags objects around and punts them.
Gravity guns are defying gravity, and every single anti-gravity experiment ever conducted has resulted in the lead scientist fleeing the country and taking on an assumed name out of shame. Most attempts deal with things like gyroscopes or magnetic levitation, concepts we understand and are no more anti-gravity than attaching invisible string to a shoe, playing new-age music and pumping a room full of dry ice. A bigger problem is in even attempting to consider the possibility of anti-gravity you’re basically arguing with Einstein. And arguing with Einstein is the scientific equivalent of lunging wildly at the Pope: you’ll get taken down.
