Characters' lip movements matching English words, even when they're explicitly stated to be speaking another language that only sounds like English because of universal translators or translator microbes.
Aliens understanding human measurements like inches, minutes or pounds.
Extraterrestrials nodding or shaking their heads to mean 'yes' or 'no', even if they've never met a human before.
Writing Kadian has been weird that way.
Kadian's natural state is a pliable, rubbery semi-liquid, which he can mold into any form he chooses. All of his cells absorb and process information together, so sensory organs like eyes and ears are purely cosmetic features, deliberately applied to set the humans around him at ease.
This makes it strange when I want to write about his physical reactions to plot, because I find myself wondering, Would he be making that expression as a learned reflex? Would he have to do it deliberately, and would he have the presence of mind to do so?
Will the scene I'm writing have the same effect if he doesn't make that face?
At moments like those, I tend to fall back on the idea that, as part of an interstellar community, his species learned to mimic their neighbours' body language to facilitate diplomacy. So while most human expressions aren't natural to him, mimicking them has become almost second nature.
Which isn't to say he acts entirely human. He does occasionally move in ways that are more specific to him, like accidentally grinning literally from ear to ear when he's so happy that he forgets that he isn't supposed to look like Cheshire.
I like movements like that in aliens, where you can understand what they're conveying, but it's still distinctly inhuman. Farscape's Scorpius comes to mind; he has a way of tilting his head that doesn't quite seem human, but it's very recognizable as a way of scrutinizing whatever's in front of him, like a predatory animal seeking a better view. It really makes his alien nature feel more convincing.
And so I watch, and I learn, and I use it to make my writing better and my aliens more believable.
Have you noticed aliens that simply move like humans in costumes? Does that bother you?
What movements have you seen people use in sci-fi, to make their aliens feel more alien?
If you've got a favorite or least favorite example, I'd love to hear about it.