Has any attack ever happened because of employee negligence?
Yes, many. For your reference, Zomato lost the data of 17 million users in a single day. This happened because of the negligence of 1 out of the 5000 employees situated in 63 different cities of India. He was a developer who used his cloud's credentials on a third-party platform (OOOWebhost). That third-party platform got compromised and the hacker stole the developer's credentials to get into Zomato's cloud, leading to a data breach.