They have to be redone if:
- They don't know how to clean the cavitations properly.
- More cleanup work will have to be done if they leave a mercury "tatoo" in your gums.
- I've read some reports of people not getting a non-toxic filling. I guess your dentist has to be somewhat trustworthy and knowledge that the composite filling you are getting is also non-toxic.
- Some fillings are not as stable as others and fallout. The ones that work best, the dentist removes your current fillings, cleans the cavitations, takes an impression of the tooth, and then bakes the new composite filling (offsite I think).
Keep in mind I'm still an amateur when it comes to replacing mercury fillings, I still have a lot of research to do before I will consider replacing my own. (I have 10 mercury fillings... which is very bad, I want it done asap).