Hey /vp/ thought I might as well warn you! Recently a few people have recieved mystery eggs over wonder trade and GTS. It's similar to bad egg from previous generations but unlike bad egg, it can multiply and replace other pokemon in the box.
If you recieve a pokemon from wt or gts and it then changes to an egg, don't check it's stats, attempt to hatch it or anything else. Either stash it away in a box and keep it there or trade it away. In the summary the egg will say 'mystery zone' instead of day care.
http://www.youtube.com/watch?v=TjsySmLeBS0 