Are there plants that produce nectar that is poisonous to either honey bees or humans?

Plants can produce chemicals in sap, pollen, nectar or honeydew that are toxic to honey bees and humans. Some plants, such as the linden, are usually considered an excellent source of nectar, although under certain specific stress conditions, they have been shown to poison bees. These “specific stress conditions” seem to occur repeatedly in most cases of poisoning and they affect the dose of poison the bees receive. When environmental conditions, especially soil moisture, reduce other sources of nectar, the bee is forced to forage from the toxic source because it is the only food available. Under “normal” moisture, other sources are available that dilute the amount of toxic substance to a level below the threshold of a toxic response.
The dose is the amount or quantity of chemical present. It is important to understand the concept of dose, because the quantity of a chemical toxin from a poisonous plant at a low dose may be a valuable medicine, while at higher dose the same chemical can be deadly. A substance that is toxic to one organism may not be toxic to another. If honey produced from one plant nectar is toxic to humans, it cannot be assumed that the nectar or honey from this plant is also toxic to bees. The converse is also possible, that nectar or honey from a toxic plant may be toxic to bees but not to humans.
Some plants that have been reported as poisonous are listed below.
Summer Titi – Cyrilla racemiflora – is toxic to honey bees and can cause the condition called “purple brood”.
Rhododendron from the heath family (Ericaceae) is poisonous to bees and humans. It contains an andromedotoxin.
Mountain Laurel (Kalmia latifolia) also contains an andromedotoxin which can poison humans.
California buckeye (Aesculus californica) – has caused losses of honey bee colonies throughout its range.
Yellow jessamine (Gelsemium sempervirens) – Humans have been poisoned from sucking the nectar from flowers or from eating honey made from this plant. Bees foraging on the flowers of yellow jessamine have appeared intoxicated, became paralyzed and died. Yellow jessamine has been reported to cause periodic poisoning of bees in Georgia and in Mississippi.
-John Skinner, University of Tennessee