Continuity mistake: In many historical flashbacks, most notably Spike's and Anya's, hair and wigs differ greatly from previous flashbacks or episodes. The director even discusses it during the commentary on Lies My Parents Told Me, saying that he doesn't understand why the wigs aren't consistent.
Character mistake: Giles picks up the potential slayer Chao-Ahn from Shanghai in Episode 7.12, "Potential". However, when she arrives in Sunnydale (Episode 7.14, "First Date"), the language she speaks is neither Mandarin nor Shanghainese (the two main languages spoken in Shanghai), but Cantonese, which is not widely spoken there. (00:08:55)Watcher