CONTAMINATION BIAS:
In an experiment, such as a randomized controlled trial (RCT), when members of the control group inadvertently receive the experimental maneuver, treatment, or exposure, the difference in outcomes between the experimental and control groups may be systematically reduced or increased1. Receiving the unintended exposure, may only manifest as a bias when it is unplanned and unaccounted for in the study design. Crossing over of participants from differing treatment groups is sometimes desired, and planned for in certain experimental designs. Also see: Crossover Experiment2, and Information Bias.
References:
1. Sackett DL. Bias in analytic research. J Chronic Dis. 1979;32 (1-2):51-63. (Link to Reference)
2. Porta M, ed. A Dictionary of Epidemiology. Sixth ed. New York, NY: Oxford University Press 2014. (Link to Reference)