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Don't we all wish we could visit Professor Willow's lab...
...to recover all the mons that we sent to him?
Like my Twister Gyarados. Long before TMs were introduced or even speculated about.
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I haven't transferred anything I wish I had back, unless you count the various evolution fodder I could have used, but didn't have room to hold onto. I only mass evolve during double exp events, so I end up having more candy than I can use in time, especially for Pidgeys. That's not important, though.
However, I do remember reading a post on this site once by someone who said they transferred their 100% Machamp because it got double steel moves. That was before TMs existed, but I still had a feeling they were going to regret that decision.