"Of course, maintaining such a voluminous list presented challenges early on—among them, a high number of false positives because of confusion about names. Some years ago, the wife of former Alaska Senator Ted Stevens was periodically delayed at airports while security officials sorted out whether she was actually Cat Stevens, the British songwriter who changed his name to Yusuf Islam and was for a while on the terrorist watch list. But officials say the NCTC has worked out the kinks in the TIDE system and has reduced the number of mistaken hits substantially."
The problem with most of the post 9-11 preventative/deterrent measures against terrorism is that it's really hard to tell if they're doing anything at all. We can't compare our world to the alternate dimension where we didn't implement these things and see what was prevented.