Now there lies logical thinking
With the information you gave,nothing will happen,if you add energised l-n then a different ballpark,
The red would be connected to the black
The brown would be connected to the blue
The circuit would be dead shorted
The mcb would operate and probably cease to exist
The customer would have a heart attack
The boss would be angry
The employee would be looking for another job
On the positive side,the electrician will have tested the circuit, found the problem,sorted it out then energised and no one would be any the wiser