What could be a consequence of operating a small unmanned aircraft above its maximum allowable weight?

Answer: Shorter endurance

Explanation: A small unmanned aircrafts are small aircrafts usually used for private purposes and it's control does not require direct human influence from within or on it, it is operated by already configured computer systems.

It is expected not to carry high weights than the specifications as stated on the manual. When filled with carrying weights above its maximum allowance it will cause the aircraft to have a short endurance (the fuel efficiency/economy will be reduced)