Having just rolled out a major upgrade to one of our flagship apps, we are currently beset by problems. Now I don't want to push all the blame onto the users, but at the end of the day, they insisted on a 2007 rollout, rather than giving us another month and rolling out around mid January, however it has meant that we have not had time to test most of the application and have to rely upon our main user to test things. However she is somewhat inexperienced in testing large wholesale changes to the system and in hindsight we have made a rod for our backs by not over-seeing testing ourselves. Sadly enough I keep hearing the comment, "Oh I didn't realise that needed testing because you didn't change it". When you have a complex application with everything interlinking, everything needs testing. Also the point of a UAT was completely miss-understood, the list of outstanding issues was reviewed and she checked that each had been fixed, but in true developer style, you fix some you break some.
Looking for the Silver lining in this one, hopefully this should mean that we get some kind of backing the next time we say "This isn't ready to be rolled out yet".