So far I have a collection of 3 shinies, all of which were obtained unexpectedly.
The first was a Shiny Aron which I hatched because I had Lairon and just wanted the unevolved form for my regional pokedex.
The second, I was looking for things on the GTS to complete my National Dex. I didn't realize the Tranquil I got in exchange for a Mantine was shiny until after the trade completed. I doubt its legit, though.
The last one was a gift. Months before Pokebank came out, I told a friend I'd give him a Shiny Ralts for free once Pokebank came out for me to transfer it, because I had left overs from chaining in Diamond version. After I traded it to him, he then said it was time to pay me back. I was really surprised when he then gave me a Shiny lv100 Altaria for free, Kalos-bred too. :)