When you breed, you watch the pokemon hatch from the egg. When Cubone comes out, it wears a skull. When Farfetch'd comes out, it holds a leek. When Vulpix comes out, it has six tails. When Ponyta comes out, it is on fire.
But these contradict the Pokedex.
Cubone wears the skull of it's mother, Farfetch'd breeds near lakes for the purpose of finding leeks, Vulpix is born with only one tail, and Ponyta bursts into flame a little while after birth.
But I think of this as more of a breeding loophole then a pokedex mistake. All of these pokemon mentioned were from Generation I, before breeding came into play, so making breeding logical was not a factor taken into account when writing the Pokedex in RBY.
...Of course, there were still eff'ups. With Kingdra causing whirlpools and Dugtrio cauing earthquakes, the Pokemon world is probably in half, lol.