This photo illustrates how a small mercury filling fracture can end up cracking your tooth.
As a metal, mercury can expand and contract in different temperatures. This, along with other pressures (biting, grinding of teeth during sleep, etc) can stress the tooth such that hairline fractures can start to appear, and then can propagate through the root.
In this case, the tooth could not be saved, since it was split in two halves down to the root. This is highlighted in the right half of the picture.
Mercury removal, or not using mercury in the first place, may have prevented this tooth from breaking.