Is this real life or fantasy: this is probably the most used one i've seen so far, a character wakes up in a mental asylum to be told that everything sci fi-y they experienced was a hullucination (buf
Evil twin/ dopplegangland: a protagonist that you love? how would they react when they see an evil twin or doppleganger? (star trek original (forgot episode but evil spock, stng: data and lore, buffy
time loop castostrophe (or groundhog day loop): time line thats ends with a catastrophe, and then restarts, only to loop until characters find a way out of said castrophic end (stng: cause and effect,
red shirts neglected: the 'extras' of a crew or team are deamed unimportant to the extent that no one ever even bats an eyelid at their death or disappearance
techno babble: something that was completely impossible before becomes possible because the plot deems it necessary and is explained by confusing techno babble (stng, doctor who)