Dan's Boss - S3-E7
Corrected entry: Approximately seven and a half minutes into the episode, Dan's new boss, Vincent, corrects Christine on a precedent she cited, then says it was overturned "On August 17, 1977. I believe it was a Thursday." Harry jokes, "It was a Tuesday." According to The World Almanac and Book of Facts, it was actually a Wednesday.
Correction: It's entirely possible that he was just mistaken, not necessarily a mistake on the researchers' behalf. Also, as you've pointed out, Harry was making a joke...