Spatial Disorientation, as it applies to pilots, can be a serious confusion that kills. There's not too many other common situations that can cause death if you get confused.

Humans are visual creatures (for pilots, that means VFR or Visual Flight Regulation :) The senses we use to keep our balance and know which end is up are wholly unreliable when we are in motion, but have no visual reference with the outside world (as can happen in IFR, or Instrument Flight Regulation conditions). Training or experience can't change natural human inabilities.

Spatial disorientation is an mistaken sense of one's position and motion relative to the earth. Basically, a condition which deprives the pilot of natural visual references to maintain orientation. Forms of cloud, darkness, et cetera or types of terrain such as arctic whiteouts and moonless skies over water can quickly bring a pilot to spatial disorientation. However, pilots have instruments to indicate directions and orientation of the aircraft relative to the horizon - although in confusing situations, humans tend to rely first on their senses, and instruments are also culpable to failure (moreso in general aviation planes). No matter the cause, it can all end with the same lethal results.

Some pilots are more trained than others to deal with situations likely to cause spatial disorientation - more specifically IFR (Instrument Flight Regulations) rated pilots. IFR Pilots are less likely to become disoriented in flight because they have undergone training teaching them how to handle the aircraft with no outside visual, and even with various instrument failures on top of that. However, this does not mean that IFR pilots cannot become disoriented, nor that no VFR pilots are capable of handing these situations.

While the physiology and dangers of this condition are taught early on in primary and instrument flight training, general aviation pilots still show misunderstandings and unconcernedness about what it is and how to deal with it. Accidents that can be attributed to spatial disorientation continue to claim the lives of many pilots (and their passengers) each year.