The variation in speech rate influences the performance of the automatic information-providing devices and leads to the recognition failures in the process of man-machine communication. While this problem has been generally recognized, there are few studies that provide detailed information on the variation in the speech rate in a specific context. Our research focuses on the variation that occurs in a clearly defined context of recognition failure, which evokes an abrupt change in verbal (and non-verbal) behavior on part of the subjects. In addition, the method we applied in measuring the speech rate improves the reliability of the results.