Back when R/S first came out, most walkthroughs and sources said that Shuppet was a Sapphire exclusive Pokemon (probably because it was so rare). So you can imagine my confusion when I was training my team up on Mt. Pyre and I encountered one. Of course I then proceeded to freak out and catch one, thinking I had some hella weird glitch going on in my game.... aaaand then the power ran out on my SP. fffff.
I thought for years I'd tripped some kind of one-off glitch by encountering so many Pokemon at Mt. Pyre (I'd been in the same patch of grass for about two hours) and it wasn't until recently I found out that Shuppet appeared in Ruby, too.