I'll go first. Mine is the instant knockout drug. Like Dexter's intramuscular injection that causes someone to immediately lose consciousness. Or in the movie Split where there's the aerosol spray in your face that makes you instantly unconscious. Or pretty much any time someone uses chloroform.
Nonsensical or thoroughly debunked technobabble. The most annoying for me is faster than light communication via quantum entangled particles. Yes entangled particles will change each other's state faster than light but this effect CANNOT be used to send information of any kind. At all. Ever. This has been known since engagement was first discovered but Hollywood is always like "I'm just going to ignore that second part." I don't even have anything against ftl comms or any other physics breaking things, just use an explanation that isn't literally impossible and well known why it's impossible for God's sake.
Better yet, don't use an explanation at all!
If you establish something as just being part of your setting that is accepted by the characters in it like it's no big deal, you can just move on with the actual plot. If it's not actually going to be relevant to anything plot wise, don't waste time with useless technobabble!
Slap a "Zephyr FTL Communications" logo on the side of the terminal and call it a day. The audience doesn't always need to know how, just what. And show, don't tell.
You can have a character exposition dump about a piece of tech that should be as normal to the other characters as a telephone (so why would anyone talk about it existing casually outside of very specific circumstances), or just... have the character use the damn thing and add a little splash screen on the device "Thank you for using Cisco Intergalactic FTL calls".
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Why?