Today a user called me asked where his file went and why SharePoint is telling him, that he can't upload it again. The error message containt a message, saying that the file is already checkout to a user.
I checked the library in SharePoint and via Windows Explorer, but the file was indeed gone. So after a while I remembered that there is another way.
Open the library settings and under the topic "Permissions and Management" choose "Manage files which have no checked in version".
And there was the file. I took over ownership and checked the file in.