As above.
Even a successful overclock is a risk, it causes additional wear and tear on components, especially gfx cards, and so reduces their lifespan. The higher the overclock, the greater the loss of life expectancy.
Having said that, in most cases, the components will outlast your use of them; as long as you are using a good psu from a reputable brand name that can handle the excessive power demands overclocked components require.
In all cases HEAT is your enemy; you need a well designed case with plenty of airflow, and much better than average cooling systems.