This article covers a subject that is part of the real world, and thus should not be taken as a part of the Holby universe.
"Teacher's Pet" is the 503rd episode of Casualty and the 9th episode of the 20th series. The cast of Casualty and Holby City join forces in this special four-part Halloween story.
As a result of only being a half-length episode, it was mistook as making up a whole episode with episode 10, which resulted in many errors in episode numbering. It was only in 2016 when Casualty aired its 1000th episode that this error came to light, and episode lists were amended.