A (beginner) player catches a pass, his momentum carries him into the attacking endzone. Some players call “not in” and “travel” because the player keeps his pivot inside the endzone. He panics and intentionally drops the disc.
If it is a beginner, then I would allow them to regain possession.
It could be viewed that there had been a discussion started by the players saying “not in”, and therefore play would stop, and no turnover is possible. (15.7)
Also if the beginner had in anyway indicated that they thought it might have been a goal, this could be seen as a goal call, and therefore play would stop, and no turnover is possible. (14.1.1)