Incorrectly configured private bucket containing critical informations like SQL backup.
Amazon S3 provides a simple web services interface that can be used to store and retrieve any amount of data, at any time, from anywhere on the web. Files within S3 are organized into "buckets", which are named logical containers accessible at a predictable URL. Access controls can be applied to both the bucket itself and to individual objects (files and directories) stored within that bucket. A bucket is considered public if any user can list the contents of the bucket, and private if the bucket's contents can only be listed or written by certain S3 users.
Cloud based storage is great but it is very easy to make catastrophic mistakes. If you are setting them up then please ensure you test the access control yourself before uploading any sensitive files.
Make sure all the Amazon S3 buckets you are using are marked as private and not vulnerable object listing vulnerability.