That's a weird problem you got there, let me tell you Oo
If you're talking about Pokémon Crystal/Gold/Silver, I'm pretty sure that old trick won't work, man.
According to some people, that glitch only used to work on the 1st generation games, like the Blue, Red and Yellow versions.
By the way, if you're doing this in one of the 1st generation games...
I know about another "Mew Glitch/Trick" that is much more efficient, in my opinion.
I got my Mew by doing that and I've had no problems so far.
First, make sure you have "Fly"
In order to perform this glitch you must not battle the gambler that's in front of a house, near Lavander Town. (Just head left of the town and you'll find a series of trainers there. The last trainer, which is the one in front of the house I talked about, is the one you MUST NOT fight.) Put yourself in front of the house and save there so you won't have any problems if the process goes wrong. walk one step further and IMMEDIATLY press "start". Choose the pokémon who knows fly and fly to Cerulean city. After the nugget bridge you'll find another series of trainers. Fight with the fifth one. He's a youngster with a slowpoke. (This is another trainer you must have not fought in the past in order to get Mew.) After kicking his butt, fly to Lavander Town once again, and head left. You'll notice the menu will pop up on his own. Press "B" to close it and Mew will weirdly appear at level 7.
If you have already fought the trainers I mentioned above, I'm afraid you'll have to restart your game.
I did that in order to get my lil' Mew and I don't regret it, but that is up to you ;D
Hope it helped. ;D