Lassooing users and putting them down to monkey around with your application may be the best way to spot deficiencies in your freshly baked program.
But it is also where some jaw-dropping things occur. Stuff you really did not expect to happen, made you facepalm for your own stupidity or the users, let us in on the fun!
My story is not one of real stupidity, but just sheer disbelief:
I developed a simple game for elderly to gradually become acquainted with a mouse, from previous users it was obvious that we needed to be clearer about that the buttons of the mouse did not need to be pressed to move the cursor, no harm no foul.
However the next user came in, sat down and got straight into it of course pressing the buttons while moving and generally having a good time. But then in the next level, 'clicking', nothing worked, he did everthing properly, I took over to try myself... nothing. No click was registering, very weird, it worked ok with everyone else.
Luckily it was the last user, and time was up. So I went home to take a look at what was wrong. Turns out the program was absolutely fine... But the man, a welder his whole life with all his strength still there, had totally crushed the internals of the mouse in the first levels... I binned the mouse with a big grin on my face.
Needless to say, I did not see that one coming...
We had an algorithm one time that brought back results of the restaurants within 5 miles of the location specified in the search. First user tester managed to pick a place with no restaurants within five miles. All out testing had been in large city locations, no small towns which might not have anything near them. We changed the formula to bring back a mininum of 5 records and if five miles didn't bring it back to extend out to ten, and so forth until we found five restaurants. After that we tested that page by putting in Eek Alaska (nearest restaurant in our database was over 100 miles away) for the search criteria. I still get a kick out of the town name of Eek.