Masking of subjects in clinical trials is important to reduce potential patient bias, but is not always so straightforward. For example, in randomized medical device trials, treatment regimes and/or device configurations are different and may be detected by the subjects. In order to determine whether the mask has been broken, subjects can be queried. Issues to be discussed, among others, include when and how many times to query the subjects on which treatment they think they received, how to interpret the results of the query, and what should be done when the subject blind appears to have been compromised.