Technically one filling may not harm someone. However, mercury is accumulative. Let's say a person with 1 filling got a flu shot every year for 15 years. The mercury in those flu shots would be building up over time and that one filling may finally push them over the edge.

Someone who never eats fish and wasn't vaccinated as a child or adult (which is pretty rare these days) may be OK with one filling. However it will begin to accumulate in the body. When you have mercury in the body, you begin to hold onto other heavy metals (lead, nickle etc).

Personally I feel any amount of mercury is bad, but unfortunately in this day and age it impossible to avoid completely.