Theorems with an extraordinary exception or a small number of sporadic exceptions

The Whitney graph isomorphism theorem gives an example of an extraordinary exception: a very general statement holds except for one very specific case.

Another example is the classification theorem for finite simple groups: a very general statement holds except for very few (26) sporadic cases.

I am looking for more of this kind of theorems-with-not-so-many-sporadic-exceptions

(added:) where the exceptions don’t come in a row and/or
in the beginning – but are scattered truly sporadically.

(A late thanks to Asaf!)

Answer

Every automorphism of Sn is inner if n6.

P.S. That S6 has an `essentially’ unique outer automorphism is quite a non-obvious fact.

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