[stiger]

Hire mercs and order their leader to become a propthet. Now let him go after three turns. If he then does not end in a merc list again - viola, you've just lost an ability to have a propthet for the rest of the game. As the merc leader was not killed the game assumes the propthet is alive and does not allow you to appoint another.

While logical, I think it could be better to strip "prophet" flag from a merc upon leaving and trigger "death of the prophet" as currently it results in some funny things like being able to command a unit that is a prophet of another god (if such merc is re-hired). This unit spreads dominion of another god but has bless effects of your god.

Seems easy enogh to fix.