Yellowcake uranium. Mobile biological warfare labs (incredibly implausible in any case). Iraqs nuclear programme. Ignoring evidence from a defector who testified to the destruction of CBW projects (yet citing other useful titbits from that defector). Failing to provide supposed WMD intelligence to the UNSCOM inspectors and blocking French suggestions to increase the number of inspectors, as well as setting the CIA to find 'dirt' on Hans Blix (apparently Wolfowitz went mental when they failed).
Hans Blix book of his experiences at the time is very interesting, and remarkably impartial for someone treated so shoddily.
EDIt; sorry, not UNSCOM. I forget the name of the new agency. Worth remembering there was both an agency and a directive for the removal of WMD without war, though, and it was sidelined with nowhere near the time it required.
EDIt; sorry, that should have been 1 post up. Top bit, which is now a mite off-context, is a list of very obvious failures that spring to mind.