A probation period looks like a formality until it is used. Either side can end the contract instantly during a valid one, with no notice and no reason given, but the length allowed is capped by law — and a clause that overshoots that cap does not do what the employer assumes it does.
Yes, and it cuts both ways — either side can end the contract instantly during a valid trial period, no notice, no reason required, no severance. Length is capped by law: no trial period at all is allowed on contracts of six months or shorter, up to one month on fixed-term contracts between six months and two years, and up to two months on permanent contracts or fixed-term ones of two years or more. It has to be written into the contract and equal in length for both sides. The trap worth knowing: if the proeftijd specified doesn't match what the law allows for that contract length — say a one-month trial on a three-month contract — the clause is void entirely, meaning normal dismissal rules apply and your employer can't rely on it to fire you without cause.