Happiest of Fridays, everyone!
Suspension of disbelief is a funny thing, especially in speculative fiction. You can make your readers believe in space travel, or ghosts, or all manner of magical powers, so long as you keep all your worldbuilding consistent and unexplained. But it's usually not the otherworldly things that can throw a person right out of the story.
Maybe your reader is from Boston, and your Massachusetts-set story refers to the subway system as the 'Metro.' (And you shouldn't do that, because I just clutched my heart in agony at the very thought.) Maybe your reader is pursuing their Masters in biochemistry, and the science in your book isn't quite working for them. It doesn't even have to be the reader's specialty. Sometimes it's just one of those tropes that rubs the reader the wrong way.
For me personally? My pet peeve is when writers fail to keep their characters' injuries consistent. I sometimes feel like I can hardly pick up a book or watch a TV show without a character talking perfectly clearly after they've just been throttled, or blithely walking around and bantering and fighting crime with a head injury or 'a few cracked ribs.' Not because I'm a sadist - hopefully not, anyway. But when the character can just glide through setbacks without even missing a step, it drains the stakes from the narrative for me. It's hard to get invested in a character when it seems they're going to achieve their goals without any problems.
(And I have a hard time understanding why any writer would pass up a chance to make their character suffer some more... which does sound a little sadistic, now that I type it out.)
Though the phrase "write what you know" is the one that gets bandied about in various creative writing classes, I don't agree with that - those would make for very boring books most of the time. But it's very important to know what you write, down to the smallest details. A throwaway line for one writer could be about something the reader knows back and front - and when a reader loses the thread of the story, sometimes it can be tricky to find it again.
What breaks your suspension of disbelief?