Apple seeds and cherry pits are POISONOUS.

Pits and seeds from these and similar fruits contain cyanide compounds. Don't worry if you swallow a few, your body can absorb a the small amount of toxins that you would find in a cherry pit or a couple of apple seeds.

Swallowing a few of these seeds is harmless, but if you happen to chew the seeds or swallow several of them your body can absorb a harmful amount of these chemicals. Normally, this would result in headache, dizziness, confusion, anxiety, and vomiting.

Consuming the cyanide compounds of seeds and pits in large quantities can cause difficulty breathing, increased blood pressure and heart rate, kidney failure, coma, convulsions, and death from respiratory arrest. Though I don't know why anyone would want to eat that many apple seeds...

Another less common side-effect of consuming too many apple seeds is wearing a saucepan on your head. Poor Johnny Appleseed needs to recognize that he has a problem.
(source)