In Classic Who, you can always tell a companion is going to leave if they start the story by complaining about wanting to go home. Unless, of course, they’re Tegan, in which case it’s just another ordinary day.
I just realised (thanks to the behind the scenes DVD documentary) that you can see the Cybermen’s organic lips and chins through the masks and I am, for lack of a better word, SHOOK. I’ve seen Earthshock at least three times, and clips from it have been burned into my brain thanks to things like Doctor Who Confidential. And I never noticed. And it was a deliberate design choice, too - they painted their faces silver and their teeth black and showed them to allude to the Cybermen still being organic beings, a bit like them having bare human hands in The Tenth Planet. (Which, ironically, was unintentional - they just lost/didn’t make the gloves!)
My memory of these Cybermen is their chins being silver and, even, reflective, which meant I was genuinely startled when I found out otherwise.