Yeah.. except for that tiny detail:

There are three conditions that must be satisfied for the defence to be applicable:

• You employer has made a statement of fact which made you believe that the money was your own;
• You acted in good faith and without knowledge of any claim for recovery from your employer and as a result, changed your position in terms of the money; and
• You were not involved in the cause of the overpayment.

So if you signed a contract for a sum of x and the employer never said they are going to pay you more, you're already acting in bad faith based on the first point. The second point is tough to argue, literally the only way to win this is if you have a verbal "contract" only and claim you never watched your bank account and just didn't notice the extra money (but then if your employer tells you about the wrong payments you have an issue again..).

In the real world you'll probably pay the money back 99% of the time, except if you want to burn bridges and leave (going after you for smaller amounts is not worth the time in court). Your professional relationship will be ruined though, which you may or may not care about.