The Sea Route 19 item 3 morphing glitch is a glitch which occurs in Pokémon Red and Blue. It is activated by scripts 0x03, 0x05, 0x07, 0x09 and 0x0B of Sea Route 19, and it will change the player's third item into a TM23 (Dragon Rage).
It is caused by glitch meta-map script activation, specifically from address D61D, which controls the scripts of that map.
Method (no Rival's effect item required)
Requirements1. A means to enter the water in Sea Route 19.
2. These two Trainers unbeaten
3. A weak team/team with Selfdestruct, to lose against a Swimmer
4. A Pokémon with Teleport or Fly
5. Fuchsia City as the last Pokémon Center you healed at (if using Teleport) or Fuchsia City unlocked (if using Fly)
Steps1. Trainer-Fly against the long-range Swimmer (see Trainer escape glitch#Long-range Trainer)
2. Return to Fuchsia City
3. Save by switching boxes (as Start menu is broken) to save the game. Do not finish the walking animation of any Trainer or object (don't battle any more Trainers, or push a Strength boulder, or use the Pewter Museum guide, or enter the Pokémon League).
4. Go back to Sea Route 19 and talk to the second Trainer. Lose to their battle to black out outside of Fuchsia City.
5. Another battle will occur, lose this one too (you may be able to win it and still have the glitch work as well).
6. Open the Start menu (mandatory)
7. Return to Sea Route 19 and you should get some walking lag. Walk down to the original Trainer but don't face them, then walk back up to Fuchsia City
You should now have a TM23 in slot 3. The original quantity will not be replaced. This has a small use (other than being able to get infinite Dragon Rage TMs), namely it can be used to remove a Cancel if it is in slot 3 and you've only ever got 1 or 2 items. This actually comes in handy for SRAM glitch if you want an ACE setup to start at item 3, because the first three items are normally Cancels, but there is another way of doing the same method (tossing all of the last TM55 which has the side effect of causing data to shift back. You may need to fix item 37 quantity and badges addresses to Fly away after this)
Method (Rival's effect glitch item)
Perform the same steps as above, but because he classes as a Trainer who walks up to you, this time you can pull off the glitch with the second Swimmer (pictured, below) instead of just the one in the water.