Can someone be put in default without having received a formal notice?

Absolutely. Some contracts or agreements may provide that either party will automatically be in default if it does not respect its commitments set forth in the contract or agreement. For example, it is common for financial institutions to include, in their mortgage agreement, a clause to the effect that the debtor, in case of late payment, will immediately be put in default to repay the entire debt. The legal consequences are the same as if the financial institution had sent a written formal notice. This is what is commonly called the automatic formal notice. However, it may happen that the person who has been automatically put in default is not aware that he or she was actually put in default. For this reason, it is often appropriate to send him or her a written formal notice to make sure that he or she is aware of the situation and that he or she can act timely.