Infinite Trainer-escape glitch loss method (Generation I)
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This trick pretty much obsoletes the Elite Four method unless you only have one Trainer and it's a long-range one. Thanks missingnofr for sharing this trick. This glitch requires one long-range Trainer/one Trainer in a grass patch to lose to and another Trainer in a different location that walks up to you.
Simply escape from the first Trainer, then lose to the other Trainer. This will activate a battle with a Pokémon or Trainer depending on the last Special stat in memory when you return to the route, and you're able to initiate a battle with both Trainers again.
Optionally, like in the normal Trainer-Fly glitch method, you can battle a wild Pokémon after losing to the Trainer to load its Special stat into memory. You can encounter a Ditto and have it Transform into your Pokémon to copy its Special stat too.