Bob and Sally are both editing story 123. Bob saves his changes first and so when Sally saves her changes she has overwritten Bob's changes without even knowing it (without performing some forensics in the story's history). This last write win is undesired.
Better would be to use an optimistic locking mechanism where a version/revision number is saved on the story that is incremented each time it is modified. This version is then checked when saving (often by providing the version in the sql query's where clause) and rejecting the update if an old version is being updated.
To revisit the Bob/Sally example:
Bob and Sally are editing version 1 of the story. When Bob saves his changes the version is incremented to 2. When Sally saves her changes the system detects that an old version is being saved and would reject with a message that the story has been updated while she was editing it. Or at least warn Sally that she's about to overwrite another user's changes--maybe with a yes/no prompt to let her confirm that's what she wants to do.
The unsupported Cloud app Who's Looking for Jira Cloud may be of interest to watchers of this issue.