When a user starts editing a post while another user is also editing, the edits of first committer is lost, since the last commit overwrites the previous ones.
Happened in this answer. See the comments.
Conflicting edit warnings already exist in the system - when 2 users try to edit the same post at the same time, and one of them gets saved - when you try to save you should get a message:
The message would be like this:
*Thanks to Arjan for the screenshot
I'm guessing either some race condition was hit - or that person might have overlooked the message.
This has happened before, at least twice I have edited a question or answer only to find that someone else has either also edited it and either my edit or their edit has been lost.
This should be a relatively uncommon event and is something I raised on meta.SO previously on my question "New Edits Posted" Banner or "Another user posted an edit in the last x minutes" while editing a question where 3 separate people edited the question in the space of 30 seconds.
It would seem that this is a time window problem, where multiple editors do not receive a quick indication of another user performing an edit. Under normal circumstances the second user should get a warning banner telling them that another user has edited the question, but if they click save before the server notifies them then there is no indication.
As more and more users become active editors I can only see this problem getting worse, and as in my previous question would recommend some kind of "catch" on the server side that notes that the question or answer was already edited in the last few minutes and redirects the editor to a page noting that an edit has already taken place and would they like to continue.