What will be the effect when flying above optimum altitude ?

Question

What will be the effect when flying above optimum altitude ?

Answer 1

If you are talking about high altitude flight of jets, the main "danger" is a reduced operating envelope.  The speed at which the plane will experience a stall buffet will get close to the speed at which it will experience a Mach buffet.  The airplane will simply run out of performance to operate within this envelope when it gets to its maximum operating altitude.

Answer 2

Generally increased fuel consumption, increased time aloft, decreased margin around high- and low-speed stalls (see: coffin corner)

Depending on the context its used, it can mean a couple different things.  The most common usage of the word is essentially an efficiency question.  There is an altitude where TAS and winds aloft are optimized, so overall efficiency is better than at any other altitude.  

There is also one usage where optimum altitude gives the best combination of fuel efficiency at altitude compared to margin above the stall.  Operating above this altitude causes the aircraft to be flown at a higher than normal angle of attack, and therefore more power must be used.  More fuel is then consumed.

The end result of both of these is that operating above optimum increases fuel burn.