It's a tie between being a stupid young kid and saving part way through the E4 with a mostly fainted team and no healing items, and being older but equally stupid and forgetting to save for long stretches then powering off before realising (generally from rage-quit against an otherwise pitifully weak trainer who got ridiculous luck from critical hits and status abuse).
Another day successfully wasted. "I need a new quote for this signature." - Me
I send Blissey, Noland sends Walrein. Walrein OHKOes Blissey with Fissure.
I send Swampert and spam Earthquake. Walrein spams Surf and manages to KO Swampert, but it's left with almost no HP.
I send Salamence. Earthquake. Walrein finally faints. Noland sends Dewgong. I attack, expecting an Ice Beam to the face. Instead, I am killed by Horn Drill.
After that, I never set foot in a Pokémon battle facility again (aside from the PWT).
Missing two attacks in a row due to Sand Veil hax and being eliminated from a national Pokémon tournament because of that was also VERY unpleasant, but it didn't traumatize me as much as the above battle.
"If you want to know what a man's like, take a good look at how he treats his inferiors, not his equals." -Sirius Black