Apple seeds contain "prunasin" which is a cyanogenic glycoside - a chemical that releases cyanide when it is digested. I believe the quantity is extremely small, so humans would need to eat a lot. Obviously since dogs eat the core and survive the amount is even small enough for some dogs.

Chocolate is also supposed to be toxic but my dog has eaten a lot of that at times (obviously not with my permission, during her adolescence) and survived thru it only wanting more.

Anyone know of any cases where either has caused a real problem?