Just recently, I caught a Ditto on Black with 28/31/29 defenses. I've been using it to build a stall team. While starting to breed a ferrothorn (I had bred 3 other pokemon previously), I gave Ditto a Power Belt and dropped him and ferrothorn off. I filled up 2 boxes with 60 eggs (my standard breeding procedure). EVERY SINGLE one of those ferroseed had a def IV of 1-10. I was worried that there was something wrong with my game/ditto/Power Belt, and I knew that I'd have to hatch all those eggs.
This happened with 4 other boxes full of eggs, all were ferroseeds.
The problem has worked itself out somehow. I'm still not sure what happened. Even without the belt the odds of getting at least decent defense is still better than 1/200 :'(
Pearl Name: Scar, 0346-6864-7749
SoulSilver Name: Alvaro, 3182-0859-8953
Black Name: Alvaro, 1893-0656-1029