Error and mistake are synonyms, you can pretty much use them interchangably. A defect however, is usually more natural, or denotes something not functioning properly because there is something missing or not working. Somone who has some kind o brain damage can be said to have a defect in their brain or a car could have a defect if it is not working well.
It would be a mistake to argue with your boss the day before he or she evaluates your performance, but to forget an important step in an assigned task would be an error.Although these nouns are used interchangeably in many contexts, a mistake is usually caused by poor judgment or a disregard of rules or principles (: it was a mistake not to tell the truth at the outset), while an error implies an unintentional deviation from standards of accuracy or right conduct ( | a mathematical error).A blunder is a careless, stupid, or blatant mistake involving behavior or judgment; it suggests awkwardness or ignorance on the part of the person who makes it (: his blunder that ruined the evening).A slip is a minor and usually accidental mistake that is the result of haste or carelessness (: her slip of the tongue spoiled the surprise), while a faux pas (which means “false step” in French) is an embarrassing breach of etiquette ( | it was a faux pas to have meat at the table when so many of the guests were vegetarians). Goofs and bloopers are humorous mistakes. A blooper is usually a mix-up in speech, while to goof is to make a careless error that is honestly admitted (: she shrugged her shoulders and said, “I goofed!”)