The Aufbau Principle, taking its name from the German for 'building-up,' is commonly used to help students work out the electron configuration of an element. The orbitals are filled according to the diagram below:
INSERT[UUPT_DMA_EU01_ima - aufbau]

The Aufbau Principle is useful as it provides correct electronic configurations for lots elements (which we meet at post-16 chemistry), but there are exceptions such as chromium, copper and palladium amongst others. However, the Aufbau Principle can cause students difficulties as illustrated in the character conversation below. 
INSERT[UUPT_DMA_EU02_cc - conversation about the Aufbau Principle and ionisation of transition metals]

 

What difficulty has the Aufbau Priciple created for the student?

The student has made the mistake of thinking that the order of orbitals implied by the Aufbau Principle diagram describes the relative energies of the 4s and 3d orbitals. She believes that for scandium, the 4s orbital is lower in energy than the 3d orbitals and is preferentially 'filled up' first. However, when you ionise scandium, the electron is lost from the 4s orbital suggesting that the 4s orbital is higher in energy than 3d orbitals. There is an inherent contradiction. 

If you choose to teach the Aufbau Principle, it is important that students recognise that it is, in fact, simply a useful tool and not a truthful representation of orbital energies. 

So what is the truth?

If the 4s orbitals are higher in energy than the 3d orbitals for the transition metals, why does scandium have an electron configuration of [Ar] 3d¹ 4s² and not [Ar] 3d³?

Reality is always complex and there is no way to 'predict' the electronic configurations of the d-block elements. Instead, they need to be uncovered through experimentation. 

The answer to the question lies outside the scope of this course, however, a great starting point is The trouble with the Aufbau Principle. The author, Dr. Eric Scerri, is an advocate of the 3d orbitals being lower in energy than the 4s orbital for the d-block elements - challenging the position taken by many textbooks.

An explanation suitable for post-16 chemistry is as follows:

• The 3d orbitals are more compact than the 4s orbital.
• Electrons entering the 3d orbitals experience greater repulsion than electrons in the 4s orbital.
• A lower energy configuration can be achieved with electrons occupying the 4s orbital before the 3d orbitals are filled.
• The reduction in electron-electron repulsion compensates for the slightly higher energy of the 4s orbital.

The two articles below are recommened reading for any chemistry teacher as they tackle some of the erroneous ideas found in textbooks and online concerning this matter.

Five ideas in chemical education that must die - part three
Five ideas in chemical education that must die - part four

You may also be interested in a blog post on the subject.