I always hear defect and flaws in the context of machines, software, and maybe a governmental system. Fault is when you do something wrong: "that's not my fault, it's his". Weakness is the word that you can use for people and companies as well: "Our company can't delivery all over the world, that's one of our main weaknesses", "When you can't tell someone to their face why you're angry with them, that's a weakness". Hope that helps. By the way, I'm searching for an English partner. Please message me if you're interested.