I deleted your post as it is off-topic to the
HGSS Help thread, but seeing you asked for an answer, it's here:
http://bulbapedia.bulbagarden.net/wiki/User:Missingno._Master/Celebi_Glitch So essentially there was a Hex glitch that allowed you to catch it but... it would be
very rare without using a hacking device and would only occur if one used a very early release of the game as well.
