Early mornings, coupled with late nights. 

Physically hard days.

Poor or restricted diet with an increased training load. 

Mental stress and prolonged concentration. 

All of these and combinations of them can lead to fatigue. Will lead to fatigue, given sufficient time. This in turn leads to micro mistakes, poor neural timing and eventually injury. 

Fatigue can hide behind coffee, sugary snacks and stubbornness but is easily spotted if you look for it.

Not recovering fully in the time you would normally expect to, regularly less than 7-8 hours sleep, using more stimulants to keep going, an elevated resting heart rate, more coughs and colds, an elevated or stressed breathing pattern, and poor focus and mental control are all signs to check. 

If in doubt, back off. Good decisions are rarely made tired and a day of down time will almost never lose the race.