Two TM24 (Dragonbreath) in the bag, obtained with this glitch
The Clair gift glitch in Pokémon Crystal (both English revisions) and likely Virtual Console, allows you to obtain two TM24 (DragonBreath) from Clair without the need of Pokémon cloning.
It was documented by member greentyphlosion on the Glitch City Labs Forums.
1. To have beaten the story (including clearing out all of the Rockets in the Goldenrod Radio Tower) up to the eighth badge. You should also have HM06 Whirlpool by this point.
2. A poisoned Pokémon
3. To have beaten Clair
1. Beat Clair and head to Dragon's Den
2. Clear the whirlpool and enter the house
3. Solve the elder's quiz
4. White out in the house due to poison, by fainting your last Pokémon
5. Enter the gym and talk to Clair to collect the first TM24 DragonBreath.
6. Return to the house in Dragon's Den to receive the second TM24 DragonBreath outside of the door
Note: According to Flameduck, this is apparently due to the game acting as if you didn't receive the TM due to already having 99 copies of them.