1. Exceptions to “Normal” Electron Configurations Due to Electron-Electron Repulsions
For example, Cr
[Ar]4s13d not [Ar]4s23d4
For Cr (Z=24), the 3d sublevel is only slightly higher in energy than the 4s sublevel. The electron-electron repulsions of paired electrons in the 4s is enough to cause the electron to jump to the sightly higher 3d sublevel.
The energy of repulsion is greater than the energy difference between the energy levels

2. Other Exceptions Due to Electron-Electron Repulsions
Nb [Kr]5s14d4
Mo [Kr]5s14d5
Gd [Xe]6s24f75d1
Cm [Rn]7s25f76d1
This phenomenon can be seen with several other elements of the periodic table. All of these elements can exhibit ferromagnetism due to their large number of unpaired electrons. In fact Gd(III) ([Xe]4f7) is used as an MRI contrast agent due to its high number (seven) of unpaired electrons.

3. Exceptions to “Normal” Electron Configurations Due to Energy Level Inversions
e.g., Cu [Ar]4s13d or [Ar]3d104s1
but not [Ar]4s23d9
As the nuclear charge grows, the inner shells contract. This creates inversions of energy levels. In the cases in which these inverted energy levels are only partially full, there will be exceptions to the normal pattern on electron configurations. In the case of copper, the 4s is higher in energy than the 3d. Note, that one might want to write the electron configuration with the 3d preceding the 4s to abide by the aufbau principle. The loss of the 4s electron accounts for the Cu(I) oxidation state.

4. The Oxidation States of Iron
Fe [Ar]4s23d6
Fe2+ [Ar]3d64s0
Most transition metals have a 2+ oxidation state although in the atom they often have more than two electron in their highest energy level, nd. The reason being is that the energy levels invert in the ion. Note, the inversion of the 4s and 3d energy levels in Fe(II). Iron has the additional oxidation state of 3+ due to the lose of the electron from the 3d that suffers electron-electron repulsion of its partner.
Fe3+ [Ar]3d54s0