Service-Public.fr replies:

The remuneration of the employee is an essential element of the employment contract. Any change in its amount, whether upwards or downwards, must be accepted by the employee by means of an amendment.

The employer imposes the abolition of premiums on the ground that the error committed cannot lead to a definitively acquired right. It also considers that the team and snack allowances which are reserved under the company agreement for employees working in teams cannot be paid to an employee who has never worked in teams. Past court rulings have reinforced his approach.

In a similar situation, however, the employee wins his case before the courts and regains the benefit of the bonuses. The employer’s arguments were not enough. The Court of Cassation considers that the error, repeated over many years, had led to the such payments shall become contractual between the parties. The employer could no longer unilaterally amend or cancel those elements.

So it's all about context. Repeated error over time, sometimes called inexcusable by judges, can reduce or even eliminate the employer’s room for maneuver.