Ask HN: What are Common Production Environment Mistakes AWS Beginners Make

When I learned AWS I made a lot of costly mistakes at the beginning such as accidentally publishing a key and leaving minimal security measurements in place. Thankfully these mistakes happened with static or non-sensitive data websites and were resolved immediately, but this learning experience made me think about experiences others have had with either their initial AWS setups or when auditing new AWS user accounts. From your experience, have you run into common security, configuration, deployment problems that tend to arise from novice users? Any recommendations for those users?