I just got incredibly lucky last night. I put a porygon with an upgrade disc attached up for trade for a porygon2. Someone gave me the porygon2 with a dubious disc attached so it evolved into porygon-z, which was lucky, but that wasn't the luckiest part about the trade. The luckiest part is that it was a shiny porygon2 so I now have a shiny porygon-z.

This is the first time I got a shiny pokemon from the GTS.