Pet peeve - authors who get simple facts wrong. Two major ones I can think of:
In "Thirteen Little Blue Envelopes" the "midnight sun" of Denmark is mentioned repeatedly. Well, unless you are in Greenland, there is no such thing as midnight sun. Very short nights to be sure, but not that short.
And in one of Lili St. Crow's novels (Strange Angels I think), she mentions "8:30pm... Or 1630 hours if you want to get all military about it." Eh.... no. Not even close.