Continuity mistake: Blaze, Gilan's horse, is male in The Burning Bridge, but in The Inkwell and the Dagger, a short story that takes place immediately afterward, Blaze is female.
Factual error: Crowley chews on a pencil while composing reports. Pencils were not invented until a huge graphite deposit was discovered in England, well after the medieval period.
Factual error: Numerous times in the series cabins are said to have wood stoves. Wood burning stoves were not actually invented until well after the medieval period.
Continuity mistake: In The Bridal Dance and And About Time Too, Will checks himself in the mirror, although later it says he can't afford one.
