Adding on to the whole thing with Gen 5's animation, this also led to Slowbro's "Shellder" only having one eye, since you *should* be able to see the other eye in certain frames, but you just can't.
I find the most odd sprite inconsistency, as far as colors go, to be Absol. It went from black/gray to a kind of sky blue with its skin. I personally prefer the black/gray, since it gives off a yin and yang vibe.