If more than one user works on the same document simultaneously, Overflow will check whether the document changed before saving any changes. If this happens, Overflow will warn the editor that someone else updated the document, giving the option to either "Save and Overwrite changes" or "Reload Cloud Document" to retrieve the latest changes.
Overflow automatically saves your work frequently, so if you choose to reload the cloud document, you will lose your edits of the last few seconds. If you decide to Overwrite the changes, you will overwrite the other user's changes in the previous few seconds; however, that saved version is in the version history.