Copper (Z = 29)
Expected configuration : 1s2 2s2 2p6 3s2 3p6 3d9 4s2
Actual configuration : 1s2 2s2 2p6 3s2 3p6 3d10 4s1
The reason is that fully filled orbitals have been found to have extra stability. Copper has the electronic configuration [Ar] 3d10 4s1 and not [Ar] 3d9 4s2 due the symmetrical distribution and exchange energies of d electrons. Symmetry leads to stability. The full filled configuration have symmetrical distribution of electrons and hence they are more stable than unsymmetrical configuration.