Boron trifluoride is another exception, that does not complete the titular structure.  
Sulfur hexafluoride is an exception to this rule because it is hypervalent; it contains 12 electrons around the sulfur atom.  Diborane is another example of this exception, because two electrons are binding together more than two atoms.  In this case, there are not enough electrons to make the entire group.  Boron trifluoride is another exception, that does not complete the titular structure.  Generally, though, atoms will combine until their valence shells contain a certain number of electrons, by either ionic or covalent bonding.  For ten points, name this insight, popularized by Gilbert Lewis.
Sulfur hexafluoride is an exception to this rule because it is hypervalent; it contains 12 electrons around the sulfur atom.  
{Octet} rule