When people use and hear the term 'Masterpiece' in common terminology it generally means a piece of work of such seminal quality it is said to be one of that craftspersons best pieces of work.
This isn't what it always meant, and frankly I prefer it's oringal meaning.
It comes from the old term and definition of a 'Master', as in a master of their craft.
A Master was designated as such when their apprenticeship had finished an they were able to create a piece of work that by their masters guaging, was of such quality they could then call themselves a Master Craftsperson of that craft.
This was sometimes many years after their apprenticeship was finished, till then they were a craft worker, but not termed by their peers as a 'master'.
So, to sum up, I view a masterpiece not as a perfect piece of work, but good enough to hold up and say 'I've learned my craft to make this and have performed all the tasks required in teh creation of it to class myself as a master of this field.
Or, 'this is evidence, as my masterpiece'.