I get the following error when I try to check out a document from a document library in SharePoint that has been recently edited:

The file "[file name]" is checked out or locked for editing by "[username]"

I don't understanding why I'm getting this error because I just recently checked in the document under the username mentioned above and it does not show up as being checked out in the UI.

How can it be that this file is checked out/locked for editing?