I'm noticing that a few pools each day return this error message; however, at the time of the message the same pool has a clone in it ready to go with an up-time of a few hours. It shows that it attempted to connect the user to a VM that was still in the customizing state. Is there any reason that it wouldn't pick the clone that was ready over the one that it provisioned at the moment the user tried to access the pool. 

Here are the finer details: 

Horizon 2209

Clients, connection server and agents are matching versions

Pool is instant clone, floating assignment, MOD min 1 max 3 spare 1