Backward transfer refers to the influence on prior knowledge of the acquisition and generalisation of new knowledge. Studies of backward transfer of mathematical knowledge have focused on content that is closely related in time and in curricular sequencing. Employing the notion of thickening understanding, we describe instances of transfer that occurred spontaneously, within experts, between topics that are typically not associated with one another in a mathematics curriculum nor taught close together in time. Further, we discuss the various activities in which participants were engaged when they realized these conceptual connections and the impacts these realizations had on our participants.