Cryptomnesia represents an intriguing type of mental illusion in which people mistakenly believe that they have produced a new idea when in fact they have simply unwittingly retrieved an old, previously encountered idea from memory. Drawing on recent research on this form of inadvertent plagiarism, we hypothesized that perceivers’ susceptibility to the illusion that other people’s responses were actually self generated would be influenced by contextual variables that impact upon the efficiency of source monitoring. The results of three studies, examining different contextual factors (i.e., Experiment 1, perceptual similarity; Experiment 2, cognitive distraction; Experiment 3, retrieval context) confirmed this prediction. We consider how difficulties in source monitoring may inform our understanding of the process and consequences of cryptomnesia in everyday life.