A bad clone is clone made from the Gold/Silver/Crystal cloning glitch which has had its data corrupted due to the player switching off the power of their game at the wrong moment.
The Pokémon which was about to be cloned instead becomes a glitched Pokémon. It usually will have a nickname with a large amount of glitch text.
These corrupted Pokémon sometimes have glitch moves and on rare occasions cannot be withdrawn from the PC. Bad clones are sometimes referred to as a glitched version of the original Pokémon; for example, a 'glitched Sneasel.' The corrupted Pokémon are also an unstable hybrid Pokémon between the original Pokémon ('donor' byte) and ????? (00) ('recipient' byte).
Bad clones can be used for the bad clone glitch, including derivative glitches known Celebi trick, Celebi Egg trick and Shiny Celebi trick; which can be used in Pokémon Gold, Silver and Pokémon Crystal to receive Celebi, or another Pokémon if the trick is modified.
Bad clones with unterminated names can also be used for arbitrary code execution.
Ways to obtain a bad clone at level 0 (beneficial for the Celebi trick)
In order to get a bad clone you should deposit more Pokémon than you have ever deposited in a box (and at least 5 or so), then change boxes and reset the game shortly after the Yes/No box disappears (Gold/Silver) or after SAVING... DON'T TURN OFF THE POWER. is fully printed (Crystal).
Getting a bad clone is normally difficult but Pokémon Stadium 2's Game Boy Tower makes it a lot easier if you reset the game after the "Saving..." message appears at one of the aforementioned moments.
To identify a bad clone, the bad clone may be female with a glitched name and become level 1 after you withdraw it from the PC.