Infinite matrices may violate the associative law
  The momentum operator for a particle in a box is represented by an infinite order Hermitian matrix $P$. Its square $P^2$ is well defined (and diagonal), but its cube $P^3$ is ill defined, because $P P^2\neq P^2 P$. Truncating these matrices to a finite order restores the associative law, but leads to other curious results. 
Ofir E. Alon, Nimrod Moiseyev, and Asher Peres