Anomalous electronic configurations
Many related questions have been asked on this topic before. I suggest you read these previous posts:

Why do elements in columns 6 and 11 assume &#39;abnormal&#39; electron configurations?

How can one explain Niobium’s weird electronic configuration?

The Aufbau Principle is a general rule of thumb to find the electronic configuration of an element but usually many exceptions arise in the d and f block elements due to electron-electron interactions. 

The concept of valence electrons does get a little fuzzy when you consider $d$ block elements since the energies of the $\ce{3d}$ and $\ce{4s}$ subshells are quite similar so I think its better to avoid it. $\ce{d}$ block elements can show a staggeringly large number of oxidation states (for example manganese which shows almost all oxidation states from 0 to +7). This can't be explained using the concept of valence electrons and the octet rule.

$\ce{Fe}$ itself can lose 2 electrons to form $\ce{Fe^{2+}}$ or 3 electrons to form $\ce{Fe^{3+}}$ so now again you can see that you can't explain this by saying that $\ce{Fe}$ has 8 valence electrons.

Other exceptional configurations: (there are quite a few):
niobium, platinum, gold, cerium, gadolinium, etc.