The Bug-Catching Contest data copy glitch allows you to make any Pokémon have the same DVs, gender, moveset and stats of another Pokémon.
It is a rare example of a glitch which only exists within the Japanese versions of Pokémon Gold and Silver. It was later fixed in other localizations of Pokémon Gold and Silver and it does not exist at all in Pokémon Crystal.
This glitch exploits the Bug Catching Contest in a similar way to the Glitch City glitch, but it requires the player to wait until a day when the next Bug Catching competition is on (Tuesday, Thursday, or Saturday) first and then to wait another two days (or modify the time via password, glitch dimension) to finish the glitch.
Access to the Bug Catching Competition
A Pokémon with Fly (or Teleport?)
If using Fly: to have beaten Chuck and obtained the Storm Badge.
A Pokémon to have its data copied.
A Pokémon with the desired species for the outcome.
Three spare Pokémon.
The day has to be Tuesday, Thursday or Saturday.
Time (to wait two days).
An empty box.
1) Arrange your party like this: Flying/Teleporting Pokémon, desired species (e.g. Charizard), three spare Pokémon, Pokémon to have its data copied (e.g. Red Gyarados).
2) In an empty box, deposit the three spare Pokémon as well as the Pokémon to have its data copied last.
3) Enter a Bug Catching competition, then immediately walk out.
4) Choose 'NO' and come back in, and then Fly away.
5) Deposit the desired species below the Pokémon that's going to have its data copied.
6) Save the game and wait until the next Bug Catching competition.
7) Return (if you enter the south entrance, you should walk through the void). The game will say that you still have 1 minute left.
8) Choose Yes to finish the contest.
9) You should have a Pokémon that's the species of the last Pokémon you deposited but has the stats, gender, DVs (including shininess), and moves of the donor.