Sittingduck, I only had one filling. It must have leaked alot though, always had this rotten taste in mouth, which disappeared magically as soon as I got my filling removed. The mercury leaking into your mouth will cause bad bacterias to flourish in there. The filling also felt like a battery if I touched another metal with my hand. There was also not much left of the filling when I had it removed.

So yes, a single filling can lead to big health problems, and I've heard this happen to others too.