This isn't really Ruby stuff but I know you guys probably have other versions. I got a Mew in yellow by a glitch. I was trading a Wigglytuff for a Squirtle on my Red version and the link cable came out! I instantly put it back in and kept tapping A. The trade worked but on Yellow where Wigglytuff should be was a level 0 Mew!
I had to put a rare candy on it to make it eligble for battle! I tried the exact same process again but it didn't work! Would someone please explain this to me!