Transylvania is inhabited by humans, vampires, and werewolves. You cannot tell simply by looking at someone what he or she is. However, humans always tell the truth, vampires always lie, and werewolves sometimes tell the truth and sometimes lie.
In addition, some inhabitants are quite insane and think all true statements are false and all false statements are true. The other inhabitants are sane and correctly distinguish between true and false statements.
So, in Transylvania a false statement could be made out of malice or simply out of honest delusion. For example, a sane human knows the earth is (approximately) round and says so, while an insane vampire believes the earth is not round so will lie and say that the earth is round.
So now we know about the vampires, humans, and werewolves in Transylvania. But there are also trolls. Of course, trolls look exactly like everyone else so you cannot tell by looking what someone is. Like werewolves, trolls both tell the truth and lie. However, a troll alternates between truth and lies. That is, when a new conversation begins, a troll may either tell the truth or lie, but then the truth of each following statement alternates.
A human, a vampire, and a troll come up to you. All three are sane. You ask what they are and one of them says, “I am a vampire. The lady is human. The other one is a troll. Neither of them is a goblin.”
Is the speaker human, a vampire, a goblin, or a troll?