Historical stuff that I know is wrong, and that I feel anyone with a basic interest in history would also know is wrong. Grr. Either a) they don't care enough to look it up / have an expert around to point out the inaccuracies, or b) they think the audience won't notice.

The entirety of the 1997-98 New Adventures Of Robin Hood series, for example. OMFG, I want the head of their researcher on a pike!