" Couldn't save chunk; already in use by another instance of Minecraft? " error happens although file isn't in use anymore

When you open a Minecraft world. Then open the save file (level.dat) with a programm like MCEdit, then close MCEdit and pause Minecraft, it will still show the error message: "Couldn't save chunk; already in use by another instance of Minecraft?" though it isn't in use anymore. This seems like the game doesn't really tests then if the file is really in use if it was once in use.