It is important to define what an error in a question paper is so that there is a common understanding and to avoid people's own conceptions impacting upon the way in which they write or check question papers. We carried out an interview study to investigate our colleagues' definitions of error. We found that there is no single accepted definition of a question paper error. There were three interacting aspects that participants considered when deciding whether a problem was an error: the manifestation of the error, the (potential) impact upon candidates, and the stage at which it was discovered.