Error "Contents lost before copy"

I am trying to append all text files in a folder to one text file, e.g.: "copy *.txt all.txt". I see some files being copied, then one not copied but the error message "Contents lost before copy", then subsequent files are copied. What is the cause of this? Of what are the contents lost? One file or all previously-copied files? This is Take Command v. 18.00.32 on Windows 7 Home Premium.