“Ignorance of law has no excuses” justify this with example?

Ignorantia legis neminem excusat (Latin for "ignorance of the law has not excuse" or "ignorance of the law excuses no one") is a legal principle holding that a person who is unaware of a law may not escape liability for violating that law merely because he or she was unaware of its content

The rationale of the doctrine is that if ignorance were an excuse, a person charged with criminal offenses or a subject of a civil lawsuit would merely claim that he or she is unaware of the law in question to avoid liability, even though the person really does know what the law in question is. Thus, the law imputes knowledge of all laws to all persons within the jurisdiction no matter how transiently. Even though it would be impossible, even for someone with substantial legal training, to be aware of every law in operation in every aspect of a state's activities, this is the price paid to ensure that willful blindness cannot become the basis of exculpation. Thus, it is well settled that persons engaged in any undertakings outside what is common for a normal person, such as running a nuclear power plant, will make themselves aware of the laws necessary to engage in that undertaking. If they do not, they cannot complain if they incur liability. Generally, a convention exists by which the laws are issued and rendered accessibile by methods, authors and means that are simple and well known: the law is readable in certain places (some systems prescribe that a collection of the laws is copied in every local city council), is made by certain authorities (usually sovereign, government, parliament, and derivative bodies), and enters into effect in certain ways (many systems for instance prescribe a certain number of days - often 15 - after issue). This is commonly intended as a constitutional regulation, and in fact many constitutions or statutes exactly describe the correct procedures.

Ex-Police officer: I'm giving you a speeding ticket. Motorist: But I didn't know I was exceeding the speed limit! Police officer: Ignorance of the law is no excuse for breaking it. Terry protested that he didn't know it was illegal to break the windows of an abandoned building, but the judge informed him that ignorance of the law was no excuse.