My group has a cleric of Tymora (goddess of good fortune) and I am considering giving them a custom magic item that ties in with the idea of luck. The item is a coin with Tymora on one side, and Beshaba (misfortune) on the other.
As a Bonus Action, flip a coin. If heads, you must apply advantage to your next attack roll or spell attack roll. If tails, you must apply disadvantage to your next attack roll or spell attack roll. The lucky coin cannot be used again until a short rest has been completed.
The item is mainly intended to allow the cleric a little more role-play in combat. My question is how can I alter this magic item to prevent it from being exploited? One possible exploit I've thought of is the PC waiting until they already have disadvantage against them, but there may be more that I haven't considered.