One big headache for historical romance is getting the slang correct. It’s 2015, and the tendency is to write in modern language. A “valley girls in gowns” regency romance won’t always work. To be accurate, it will work some of the time, but not always.
I’ve been using a pretty darn good online etymology dictionary to check on usage. But it’s manually curated. Therefore it makes mistakes (as do troll reviewers). An example from a book that is in the final editing.