What recurring tropes in SF annoy you by their silliness or unscientific assumptions?
I guess this mostly applies to movies and TV series, books usually have a higher standard, but feel free to include something literary if you want.
I'll start:
1. space battles with sound-effects, in a vacuum. They all do this. Bugs me every time.
2. use of "light-year" as a unit of time.
3. confusion of star-system and galaxy.
Numbers 2 & 3 are not so common now, but used to be very widespread.
As for point 1. That sound in vacuum does not bother me as much as battles themselves. I always think of it as some kind of sound imaging system (just like proximity sensors in cars). Hearing is after all another sense that can be used as additional data delivery channel for humans.
What bothers me more is side against side engagements of capital ships (it's like XVII century in naval warfare again) and II WW dogfighting. If anything, real space combat would rather be like todays naval engagements (and more likely submarine ones) with key factor being the ability to detect enemy before he detects you.