As a beekeeper, honey harvester, and honey enjoyer... I offer a honey tidbit of the day on the subject below. I note that unusable honey is VERY rare, and as far as I know, not a concern here in the US.
There are a few rare places where local honey is actually poisonous, via poisonous pollen. In these localities (I don't know of any in the US), bee keepers only collect harvestable honey outside of the problem pollen-producing timeframes... or don't collect at all. I believe there is someplace in Europe (Holland?) where a beekeeper is required to determine there are no poisonous-pollen producing plants within a certain radius of their hives.
Ive often wondered about honey from oleander blossoms.............