There's no denying there are regional accents and dialects and even sayings, but to nitpick the examples I gave re Mad Men is missing the forest for the trees, as the point is still valid that it's easy to get the vernacular wrong since it's constantly evolving.
Heck, I've asked people from other countries and a regions to go over dialogue I've written for contemporary characters just to make sure I'm not betraying said characters by making them speak incorrectly.
Back to the topic, though, all TOS fanfilms fall into this trap.