Lilies

Although this fact will make you smarter, it may also save a life.  Lilies of almost every single variety are extremely toxic to cats.  Depending on the type and the exposure, it may cause renal failure and eventually death.  Unfortunately, this is not common knowledge, though it should be to all cat owners; even well meaning gifts of flowers could have potentially dangerouse repercussions.  There are other plants that are toxic as well, and you should know about them.