A user was attempting to edit a SharePoint Standard 2010 hosted Visio file that had been uploaded to a document set. The document would automatically open via Visio's IE plugin. The user could then modify the file and attempt to save it but would receive the error:
The document is already open for editing by someone else.
As if the document had been checked out. But I had just edited and saved the document on my own system no more than 10 seconds before where it had opened inside the full Visio application, not the browser-hosted, plugin version. Just to be clear, this is not Visio Services related.