It is often referred to as 'anomalous electron configurations' or 'unusual order of filling of orbitals' of a few elements e.g. Cr, Cu, Nb, Mo, Ru, Rh, Pd, Au etc.
The Wikipedia article clearly states :
[n+l rule] was suggested by Charles Janet in 1928...Janet "adjusted" some of the actual n + ℓ values of the elements, since they did not accord with his energy ordering rule, and he considered that the discrepancies involved must have arisen from measurement errors. In the event, the actual values were correct and the n + ℓ energy ordering rule turned out to be an approximation rather than a perfect fit...