@britzl – your guess was right 
I had a few different characters managed by the game logic script and had not initialized one of them to reduce noise via debugging. The game logic script still posted to that character’s instance id, which since it hadn’t been initialized was nil. This caused the loop.
As long as all of the instance ids for characters that the game logic script post to are not nil, it is working fine.
Thanks @Ragnar_Svensson as well for your suggestion. There is a game object that owns the factory that is used to spawn the character instances, and messages are sent to those instances.
Cheers,
Alex