Most common causes
* You have violated a business rule and are using similar data from one virtual user session to another, particularly on user credentials
* You have not correlated an item related to session, state, date, or time
* You have missed dynamic data related to the business process which is no longer valid in the system
* You are not checking for expected results and not branching your execution when the expected results are not present
* You have continue on error enabled without branching in code