After Finally getting a Generation IV game and a DS, I go to play about with some of the new stuff, GTS being one

After gaining pokerus by some sheer amount of luck, I decide to go EV train but have had no luck getting a burmy (to get the macho brace) so i consult GTS to try and get one, none availble with the pokemon I have, So I throw up a Porygon with Upgrade hoping someone will take it

I then find one by myself, So i go to take the porygon back....And it evolves...

Confused I try the same thing with a haunter (putting up to trade, then taking it back) Nothing...

Anyone else know why this happened and if its a known thing, are they're any other pokemon this works for?

Yeah man, this will only happen when a trade is completed while another Pokemon is on offer. I used this glitch to evolve my pokes ie. Magmortar, Machamp, Alakazam and all other pokemon that require a trade to evolve. Have fun.