Aye well, according to Netflux's Outlaw Prince, Robert the Bruce occasionally said 'OK'.
If you go back that far the language is going to be basically unintelligible to a modern audience, so I'm reasonably happy to overlook such things, as long as they're not too obvious newspeak. I'll tolerate "OK" as normal and ordinary part of the English language (though presumably older viewers may consider it a conspicuous Americanism), but Robin Hood talking about "human trafficking" in the recent BBC series grated.
(The
Deadwood approach of substituting contemporary profanity of context-appropriate severity for what would have otherwise come across as mild/obscure blasphemy deserves honourable mention for effort, though the end result was equally cock-suckingly distracting. Cock
sucker.)
It's the 20th century stuff where this is pure sloppiness which is more jarring. If it's pre-war Britain
[1], you're not allowed to us USAnian car terminology unless the character is actually an American.
Bonus points to anything getting it wrong where cultural difference is part of the story: A while ago I watched an initially intriguing mini-series a about a generation starship supposedly launched from earth in the 1960s. The plot hinged on the crew having been out of contact with Earth since the launch, with their culture and technology developing in different directions. Which made the use of contemporary computer jargon stick out like a sore thumb. At least until [soilers] made that the least of the problems.
[1] USAnian writers projecting their own culture onto a Britain they have limited experience of is a separate, and even more hilarious, phenomenon.