By my reckoning screenwriters, location scouts, art directors, costumers and set decorators should all know a period well enough to ensure authenticity. It's, however, acutally up to the continuity clerk or script supervisers to make sure glasses don't fill or empty at random,clock hands don't shift hours in the space of seconds, or peculiar things like that. Somehow blunders do creep in - check out Bill Givens' Film Flubs books for some hilarious moments.