The great difficulty in thread debugging is the lack of predictability and certainty in application behaviour. For a single-threaded application that is given the same application input, the behaviour will be the same for every execution, which makes tracking down unexpected behaviour quite easy. For a multi-threaded application, behaviour can be different even if user input is constant, because the order in which threads are scheduled for execution will vary each time the application is executed, potentially causing a much wider range of possible behaviours for the same input.