Three experiments investigated whether a process akin to Kamin's (1969) blocking effect would occur with human contingency judgements in the context of a video game. Subjects were presented with sets of trials on each of which they could perform a particular action and observe whether the action produced a particular outcome in a situation in which there was an alternative potential cause of the outcome. In Experiment 1 it was found that prior observation of the relationship between the alternative cause and the outcome did indeed block or reduce learning about the subsequent action-outcome relationship. However, exposure to the relationship between the alternative cause and the outcome after observing the association between the action and the outcome also reduced judgements of the action-outcome contingency (backward blocking), a finding at variance with conditioning theory. In Experiment 2 it was found that, just as is the case with forward blocking, the degree of backward blocking depended on how good a predictor of the outcome the alternative cause was. Finally, in Experiment 3 it was shown that the backward blocking effect was not the result of greater forgetting about the action-outcome relationship in the experimental than in the control condition. These results cast doubt upon the applicability of contemporary theories of conditioning to human contingency judgement.