No, not at all-I would take the view you would be able to justifiably explain why you wouldn't have noticed it-especially if there are large volume of similar transactions on the account?
But that doesn't entitle the customer to keep the money.
Do not underestimate the long memories of banks and their employees.
Customers who profit from a bank error / honest mistake / human error and refuse to repay what they genuinely owe will find that the same bank may do them no favours next time they look for a loan / mortgage / general assistance.