As a teen, my best friend and I used to fish a special river every summer- my family for go on vacation for 2 weeks, then his and we'd have a couple of days overlap... and he and I would get 4 solid weeks on the water.
We started a personal "One Fly" competition and would flip a coin to determine who went upstream and who went downstream, and we'd fish until we lost the fly or for a set period of time, then compare our catches (C&R after 10 trout, honor system).
The second year into this, we decided to create our own pattern, and fish only that fly. That year, my fly out caught his in number and size and all of my fish were natives, namely German Browns. The next day, we mixed it up- he gave me his fly, I gave him mine... and the results were the same.
So yeah, for me? It was the time a fly I created caught the most fish in a "One Fly" two consecutive days in use by two different fishermen.