Given a sequence of structures following some pattern, one loosely calls isomorphisms that pertain to some special elements in the sequence, without themselves following that pattern, exceptional or sporadic.

Examples

Spin groups

The archetypical example is isomorphisms of spin groups. These appear in the infinite sequence $\big\{Spin(p) \vert p \in \mathbb{N}\big\}$, and for low values of $p$, but not generally, there are isomorphisms to other classical Lie groups.