We are supposed to work in local files. Occasionally people will fire up Revit and end up working in the central file instead. These days it is more deliberate because they have to un-check the Create New Local option to do so.
Revit has been tweaked over the years to deal with central file editing. I've read claims in a couple books recently that working in a central file prevents other users from using Synchronize with Central (SwC). It isn't true in my experience. If I'm working in a local file I can use SwC as much as I need. The person working in the central file on the other hand will be forced to create a local file when they try to use SwC. They'll get this message.
I can work in a central file as long as necessary, as long as nobody else is working on the project in local files too. Isolation is fine, collaboration is where the conflict arises. We encounter the message above when a change has been made via a local file while the central file is being edited. While someone is editing the central file a local file user has been able to add or alter an element and then use SwC. When the local file user completes a SwC and the person working in the central file attempts to use SwC the central file editor will get this message (remember Save is disabled in a Central File).
The message also explains how to resolve the situation, just Save As to save the file with another name, which turns it into a local file. Now SwC will work.
There are some legitimate reasons to work in the central file, just do it in isolation.