Is the poison in Hamlet that was poured through Hamlet's father's ears which curdled his blood and cracked his skin a real poison?
King Hamlet's ghost identified the poison as Hebenon (or Hebena in earlier Shakespearean editions) to his son. Whether or not Hebenon actually exists or not is a source of debate.
There isn't a specific poison called Hebenon per se, but it has been argued that it could perhaps be yew (as the symptoms match up), maybe ebony (because of the spelling at the time), or henbane (because of its toxic nature and somewhat similar name).
However, there is definately also the argument that Shakespeare just made it up and may not have been familiar enough with botany to have picked a specific plant.