One extreme example would be that if I wrote a contract requiring you to become my literal slave if you didn’t pay your subscription on time, you would in practice suffer no consequences from not paying, because the contract itself is illegal in most jurisdictions. A less extreme example would be that where I live (Northern Europe), I can sign a contract saying that I waive my right to sue the company I purchase a service from, and then sue them anyway because it’s my right by law and asking me to waive my rights is an illegal contract. Or that where I live, non-compete clauses in employment contracts are illegal unless you offer 100% salary throughout the non-compete timeframe, so I can sign an employment contract with unpaid non-compete clauses and just ignore that part as it’s illegal.