Sometimes you may want to add a content editor to a document library page http://server/Shared%20Documents/Forms/AllItems.aspx?PageView=Shared and might want to move it above or below the document library and might get the error below
"The file is not checked out. You must first check out this document before making changes."
This is because in the "Versioning Settings" of the document library settings the item "Require documents to be checked out before they can be edited?" might have been set to "yes". Though you may wonder that you were not editing any documents but just trying to move the document library web part around in the page and it still would not allow you to make any changes before you turn the "require documents to be checked out..." option to "no"!
Once you have moved your web parts around you can go back and change the versioning settings back to how it was.