Percy's Big Mistake is an episode of Thomas and Friends, the long running children's series about talking engines on the Island of Sodor, and Percy is the small green engine at the centre of the story. The line comes from Percy's worry about getting things wrong and running behind, which is why it has been lifted from the show as a standalone reaction clip online. Shared on soundboards and in short edits, it now stands in for any lateness panic, far beyond its original children's television context.