Our research in the last decade has been into classroom situations that we perceive to make demands on mathematics teachers' disciplinary knowledge of content and pedagogy. Amongst the most visible of such situations are those that we describe as "contingent", in which a teacher is faced with some unexpected event, and challenged to deviate from their planned agenda for the lesson. Our findings and the associated grounded theories have been open to enhancement and revision as new classroom data has been gathered. In this article, we propose a classification of the origins of contingent classroom episodes: namely the students; the teacher him/herself; and pedagogical tools and resources. This classification extends and elaborates our original conception of "contingent", in response to more recent data.