A buddy of mine told me this one:

His crew had to cancel a revenue leg due to maintenance and wound up ferrying the plane to a maintenance base. There was a DHing crewmember that was going that way and hitched a ride. The flight attendant decided to take a nap in the back since there were no passengers onboard and didn't see the DHer board the plane. The Capt. and FO had the DHing pilot come up into the cockpit and fly the plane and had him lock the door behind him. Then they woke up the F/A in a panic to ask her how to unlock the door from the outside because they were locked out. They let her go for a couple minutes before they told her there was another pilot onboard.

Also with new FA's when they call the cabin, answer " scheduling ... what do you need?" Had one actually say sorry, wrong number then hung up and called us again