In the show when Ash, Misty, and Tracey come back to Pallet, Ash wants to fight Gary. Brock said that Eevee's coat was shiny, and that Gary did well raising it. Tracey said he thought Eevee was ready to evolve into Jolteon, Flareon, or Vaporeon at any time. Gary has to use a stone to evolve Eevee.
In the game, Pokemon require a stone to evolve. In the show, Pokemon evolve with a stone or when they've gained enough experience. So, this is not a slip up.
Okeedokee!!!
First off, the fact that a Pokemon is "ready" to evolve into its stone-evolution form does not imply that the Pokemon will evolve into that stone-evolution by itself. When Tracey mentioned how Gary's Eevee was ready to evolve into a Jolteon, Vaporeon, or Flareon, I think what he meant to say was that Eevee appeared to have possessed the determination and drive to become one of those three. In the game, we don't really have to worry about Pokemon's feelings (other than happiness, but that's another story) and we can "force" evolution upon our Pokemon. In the cartoon, the Pokemon's feelings are an issue. Bulbasaur seemed ready to evolve into Ivysaur, but Bulbasaur didn't want to evolve. Lt. Surge's Raichu had evolution forced upon it.
My conclusion: Gary's Eevee possessed the drive and determination to evolve, so it was "ready" to evolve into one of the three types mentioned above. Because it possessed the drive and determination, it was ready to have a stone used on it without any objection. That, in my opinion, is what Tracey meant.
There is no slip-up. I a later Johto Episodes, Gary has a Blackki, an undubbed Umbreon. Just check out Bulbagarden.com, there's a clip! So, Tracie was *right* for once. I hate him. :)
He gets an Umbreon? Oh no! That was such a major spoiler!
Anyway, Tracey stated that the Eevee was ready to evolve into a Jolteon, Vaporeon, or Flareon. Tracey said nothing about Espeon or Umbreon, so your saying Tracey's right is incorrect.
Tracey is not an expert (compare him to prof. Oak, now that's an expert). He just studies Pokémon and sketches them, and he wants to become an expert like Oak. He may have been wrong.
How about in 'The March of the Exeggutor squad' where Exxegute evolves by itself. So Maybe Gary's Eevee can evolve by itself. Remember the game is just a game. The cartoon tends to be more interesting and realistic so you might see more stone evolution Pokemon evolve by itself or even trade evolution. Brocks Geodude may evolve into Graveler then into Golem by itself without trading. So this may not be a slipup.