I hear so many times over frequency "cathay.. uh.. xxx" and always wonder if they have some kind of reason why pilots cant seem to memorize it.
Commercial pilots rarely fly the same flight number twice in a row, they may be on flight 010, then 218, then 037. Commercial airplanes have a place to put the flight number in the cockpit so the pilots can read it, sometimes a pilot will start to transmit and realize they can't remember the callsign and they have to look at the piece of paper, which is why they pause.
The same is true with GA airplanes, I belong to a flying club with 10 different planes, each with a different registration, and I may fly any one of them on a given day. The registration is generally placarded on the dashboard, I have more than once started a call and had to refer to it.