Before we can talk about the exceptions to the electron configurations, it's important to understand why they exist. We're going to say here that P&D, subshell or sublevel orbitals are most stable when they are half filled or totally filled with electrons because of symmetry. So here for P&D when they're half filled they're stable in this way.
So remember following off bound principle, Not off bound principle, but the following the Huns rule, electrons that are degenerate or have the same energy, they're half built, so up, up, up up up up up. So when P sub levels or subshells are filled halfway, they're stable. When D are half filled, they're also stable. Now when they're totally filled in, they're also stable.
So up, up, up and come back, around, down, down, down, up, up, up, down, down, down, down and down. So just remember it's this drive to either be half build or totally filled by the P&D subshells or sub levels that causes some of these exceptions to the electron configurations that we'll see later on.