Uhh, well, this story involves both me and a younger cousin of mine. Happened a few years ago (DPPt)
So, he asks to borrow my arceus to beat the Sinnoh E4, right? Except he didn't give it back.
That was rather aggravating, except the pokemon I borrowed from him was a gabite. I think he already had a garchomp, it just wasn't leveled enough.
Cue me partially EV training it (partially because he had used it before, and I forgot the numbers of EV training), and siccing it on his battling team a few weeks later. It took out EVERYBODY but the arceus (We were playing on PBR).
The icing on the cake? His gabite had EQ already on it, so I didn't have to waste my TM on it.
I guess both of us should be face palming, me for losing an arceus to him, him for giving me a gabite with EQ on it.