I re-read Dirk Gently's Holistic Detective Agency the other day, and was rather startled to find what appears to be a genuine continuity error. In most books this wouldn't be cause for remark, but this book in particular seems so careful about its continuity the rest of the time (particularly given the nature of the story) that it was quite a surprise.

Sarah, the little girl we encounter being bored at a formal dinner at the very start of the book, has blonde hair when described by the narration, but when Dirk gets a description of her from Richard later on it says she had dark hair. Gosh, ooh etc.