Has any attack ever happened because of employee negligence?

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.