This glitch only works 'properly' on the Japanese versions of RGBY. Due to different memory addresses being hooked, you can only catch a metapod in the International English versions.
By the way, this Mew...You can also buy Master Balls for free using a variation on the same glitch.
Old School Fishing
* 1st Gen glitch technique.
Caught it at the Celadon Department Store, sonny.
Shop fishing glitch, only works 'properly' in the JP versions of RGBY.