The UK Border Agency Employee Codes are specific for the employees and thus you can easily get the employee who had stamped the documents. This would help to minimize the amount of time required to track down what has happened.

Yes, some people having some bitter experience and in order to square their act they resort to such activity. This is indeed a very old issue and much has been talked about this. I think that such incidents do have a bad effect of the employees in general among the employers.