Ah man I've had a few of these moments in the past...
Emerald- Friend at school was showing me the cloning glitch. I attempted it at home with my Colosseum Ho-oh and XD Lugia, but I forgot the steps and did it back to front...They ended up disappearing instead of being cloned.
Another; waking up after a sleep over to find someone had traded my legendary dogs while i was sleeping.
Oh and eventually my emerald game just got stolen along with my ds altogether.
Diamond/Platinum?- Was extremely tired in a car. Was trying to sleep and EV train my magmortar in the old chateau. I'd memorized where all the moves were so I would just keep pressing where the skill was when i encountered a battle, all while my eyes were closed. Randomly decided to open them at some stage and noticed a shiny gastly just chilling there while its hp slowly dropped to zero...Yeah that was a bummer. This happened again i believe, only with a different pokemon, but I can't remember what it was...Its all good though, I found a shiny bidoof eventually...
That's all I can remember from the top of my head but yep had some bad experiences.
friendly code is:
platinum (i think...): 4597 8555 9517
Soul Silver: 0947 5117 8702
Black: 0604 5929 3965
3DS: 5112 - 3439 - 6831 (Add me all you randoms. PM me your fc if you add me.)