As a consultant in interaction design and usability assessment, I conduct a fair amount of usability testing . The kind of testing I do generally involves preparing test scenarios in advance, because my clients and I want to make sure we test the aspects of greatest interest and concern, and that we test them the same way with multiple participants.

So I was testing a web site that allowed users to make travel reservations. In exploring the site to tease out major issues, I noticed that it didn’t handle cities very well by name; it seemed much more comfortable with airport codes. So I chose to have participants make a reservation to Lincoln, Nebraska, figuring that they might guess LIN as the airport code — and end up in Milan, Italy. (Lincoln’s code is LNK; LIN stands for Milan’s Linate Airport.)

Most of the participants in the test had at least some trouble with booking a trip to Lincoln, and more than one actually did wind up with a reservation to Milan. When one of these people realized what had happened, she howled with glee, then turned to me and demanded, “You did that on purpose, didn’t you? You knew that would happen!” (At this point I was free to laugh too, and I had to admit that I had.)

That’s my funniest moment in usability testing. What’s yours?