The problem is the residual magnetic effects left over from the original writes.
That is why the ONLY safe method (other than physical destruction) is multiple writes with different patterns. It seems that about 7 re-writes gets one pretty close to being unreadable.
Pretty close, but still not unreadable. I remember reading a paper from a fellow who was, using an electron microscope and some other exotic equipment, able to recover data that had been randomly overwritten as many as 30 times.
The equipment and techniques exist, AFAIK, to recover data regardless of the pattern or number of times it's overwritten. Obviously, though, very few people have information on their hard drives that make this sort of effort worthwhile, but the only way to really be sure is to physically destroy the drive and platters.