Final answer:
A learner can sabotage group contingencies by engaging in social loafing, which can be reduced through strategies like flocking and individual assessments.
Step-by-step explanation:
A learner could sabotage a group contingency by engaging in social loafing or the 'free rider' problem where they do not contribute to the group's work but still benefit from the outcomes. This behavior can lead to negative consequences for the group, such as a failing grade if every member adopts the same strategy. To combat this, strategies like flocking to form groups with similar schedules and motivations, or implementing individual assessments, can reduce the incidence of social loafing and improve group outcomes.