I have heard someone tell a story about that happening once. They said that the cabin crew announced that they would be playing a game; they would call out an item, and give a free drink to the first person who could find that in their carry on. The first couple of items were fairly standard things; "a fountain pen", "a camera", etc. Then they got to "a commercial pilots license". Someone actually did raise their hand, and the cabin crew escorted them up to the cockpit.
There was never any problem with the flight, and he never found out what happened. Probably something like the co-pilot had a problem that prevented them from continuing to act as the co-pilot, and they wanted to maintain sufficient redundancy just in case.
The flight time requirement for a commercial pilot is only 250 hours. All flight instructors are commercial pilots. They are more common than you think.
Also, isn't there a pretty big distinction (even just in terms of required flight hours) for Commercial versus Airline Transport (ATP)? ISTR that ATP required at least 1k hours.
There was never any problem with the flight, and he never found out what happened. Probably something like the co-pilot had a problem that prevented them from continuing to act as the co-pilot, and they wanted to maintain sufficient redundancy just in case.