I don’t know that it’s an actual issue, but it’s something we encountered while working in “Co-Authoring” mode with others. This was in an Office 365 SharePoint Team Site to give you a little context to this.
Ideal for us as most of us do not use a PC, working on a very large document (currently 7500 words in this Word document) with Office Web Apps was ideal.
All of a sudden and this was happening of course around 5pm, Word Online did not want to save the document anymore with the following error:
Sorry, your changes weren’t saved. To continue editing this document, please copy your changes and refresh the page.
Naturally, I assumed it was a temporary issue perhaps with SharePoint or Office 365 and decided to start again tomorrow.
Next morning however, I still got the issue. Now don’t ask me why I checked the versioning, but I did and well take a look for yourself:
We couldn’t get past version 0.510!
Mind you this could have absolutely nothing to do with it, but when I published a Major Version of the document and went back to 1.0 everything was fine again. And we are continuing to work on the document.
Looks like it was a Software Limit by SharePoint (Thanks Wictor Wilén) the minor versions limit is indeed set to 511 https://technet.microsoft.com/en-us/library/cc262787.aspx#ListLibrary
In any case, it’s always good for you to perhaps set some limits for the versioning especially for the minor versions regardless if this had anything to do with the problem mentioned above.