This doesn't count as a 'release' per se, but I once put my Jirachi from HeartGold into the Pokewalker to level it up and... lost the Pokewalker. I was very annoyed at myself for losing such a rare Pokémon.
Guess I can mention that I've released several good IV Pichu with Volt Tackle during a time that I attempted hatching a Shiny VT Pichu? I wasn't (and still am not) a competitive player and lacked WiFi to trade them off in D/P/Pl, so.. but yeah, I can feel the cringe from that
I once released a shiny Pokémon as a joke to my friends... I forgot to turn it off and back on afterwards and saved it... sad to say that I actually miss that eevee.
I accidentally released my starter in Red once, I didn't save so I could get it back but I lost some story progress and levels which still made me sad back then since leveling was a pain.
I pressed 'Release' to see what would happen on this high-leveled Breloom. Enough said. I released a few egg move Torchic because I had no Wifi and was impatient. I regret that now... #MissingNo
I accidentally released my Relicanth in Sapphire. I was mad, because I was trying to unlock the Regi caves, and hadn't saved for a while (At least before I had been leveling up Wailmer to Wailord from Level 30). So I had to go explore the ocean depths to find another. >_>
Ugh... the nightmares... stupid Relicanth... >_< I can't remember any accidental releases I've done, mainly because I *NEVER* release a Pokémon. ;;;
I once accidentally released a magikarp I was planning to evolve that was holding an exp. share I wasn't even paying attention when I did it
