In the summer of 2009, I was contracted by CVL to conduct user testing of new basket and checkout designs that CVL had created for a long-term client of theirs - Argos.
Argos of course have a unique place in the UK retail market. Despite Bill Bailey's "The Laminated Book of Dreams" lampoon, they were there first with many novel operational and marketing practises: electronic inventory and warehousing, order online and pick-up in store are two examples. Their unique position means that they do things in a very particular way, which inlcudes the design and testing of their website.
CVL, it turned out, had been designing, and then testing, Argos web page designs for some timne: not a generally well-considered practice as, of course, the solution designer will always introduce bias into the solution testing.
So when I attempted to explain the testing results (with recourse to the notion of a mental model) by pointing out that neither CVL (the designers) nor Argos (the client) had a full or proper grasp of what it was that their customers thought and believed about the checkout process - well, let's just say that the final results presentation was a challenging couple of hours!