... could be caused by cheating

[Nikelaos]

i have found that if someone buys a merc and then sets it to build a fort but someone else buys the merc before the fort is finished then the merc still has commands to build a fort, changing the command to something else results in the player being given the price of the fort into his coffers, this tends to cause the cheat message to appear though it may have been fixed in recent patches.