On a recent project I was working on and we had a requirement for reports to be displayed online.  We had a discussion about the format of the report and we agreed with the customer that we would need to provide sorting on the columns.  This was fine but when it came to implementing this we did not think it through properly.  So what we ended up with was sorting on all columns alphabetically when what was required was sorting by date and by amount on some of the columns.

This was reviewed by the customer but never picked up a them.  What I haven't mentioned is that we never had any UAT testers other than the customer contact who was developing the specification for us. So although we got feedback early we didn't get all the feedback we needed.  However I'm sure that if users had started using the application they would have picked up on this error.

So it seems that feedback is best when it comes from production usage of the software as this is when testers (or users) care most about what works and what doesn't.

Hopefully in future projects we will have a more iterative development cycle and get earlier feedback from production or UAT users.