The exposure of the errors on the Criminal Records Bureau database and the effects on the lives of ordinary people is a classic example of the damage that "false positives" generated by this kind of system can do. Nearly 3000 people were affected, some turned down for jobs and the like. And the CRB system is only a tiny fraction of the size of the proposed national identity register.
Add to that the serious questions raised by the case of former police officer,Shirley McKie, demonstrating the way in which things can go wrong in the processing and analysis of fingerprints.
Two important parts of the national ID card system involve maintaining complex accurate records of personal details and fingerprints. They are just two amongst many, however. Now how confident can we really be that the Home Office are capable of managing those two parts let alone the rest of the system?
(BTW for the ID geeks - I know it's not as simple as separating just two items out like that but I'm just trying to illustrate the point)