Olde Tyme Language
Well, not quite that bad…
Still, having read a plethora of fiction set in a variety of time periods, I can tell you, one of the biggest mistakes modern writers make is using modern language in historical settings. Or, worse, completely misusing historical language. This seems to be an especially large problem for new or amatuer writers of fantasy or historicals set in the pre-1900’s. Luckily, C. J. Cherryh has an answer for those poor unfortunate souls: Pre- and Post-1900’s Language. It’s far from complete, which she admits herself, but it’s certainly better than nothing and it is a great place to start.
Also, anyone looking at creating their own world, with its own linguistic twists on English, would do well to look through this file. It’s a great look at ways that English usage has changed, especially in terms of slang and common usage.