CoveTom said:
^ In "The Mind's Eye," when Geordi is getting ready to attack in the cargo bay, you can hear Picard talking in the background. I believe he says something about the cargo transporters always introducing a minor flaw or somesuch into the cargo it transports so that it can always be traced back to its source.
With cargo that would work fine, but I would think that if a person tried to transport through a transporter so programmed, a seemingly minor imperfection introduced could have dire consequences.