Ok I can reproduce that bug now.

Create a filefield. Upload the same file a few times.
Then on a node, click edit=> save (you do not need to save anything) => edit => save etc...

Then you will see that error message : 'The selected file could not be copied, because no file by that name exists. Please check that you supplied the correct filename.' and in your file directory , one of the files is missing (the file_n with the highest n).

Tao