When uploading a document at the same time as someone else, the second person to start the upload may get the first persons document if they hit finish first.
TitleDocumnet manager Upload issue
This behavior is expected because all of the uploaded docs are stored within the same temp folder for the same SharePoint list. After the docs get inserted into the Rich Text Field, and the qListForm is saved, the files will be removed from the temp folder. So if user A is working on adding some docs to a list, and user B is trying to do the same to the same list at the same time (before user A saves his work), then yes this behavior is expected.
There has been an ER (TF00264494) submitted to change this behavior.