Depending on a standard of duty which a party has agreed to assume under the contract he may find himself liable for breach even "without what one might call fault on their part".
Moschi v Lep Air Services Ltd  2 All ER 393, per Lord Diplock (p.400):
The law of guarantee is part of the law of contract. The law of contract is part of the law of obligations. The English law of obligations is about their sources and the remedies which the court can grant to the obligee for a failure by the obligor to perform his obligation voluntarily. Obligations which are performed voluntarily require no intervention by a court of law. They do not give rise to any cause of action.