Datafile seems empty, but dropping raises "ORA-03262: the file is non-empty"

 Trying to drop a datafile:

   SQL> alter tablespace prod drop datafile 13;		
		
   alter tablespace prod drop datafile 13	
		
   *		
		
   ERROR at line 1:		
		
   ORA-03262: the file is non-empty
	
   SQL> select * from dba_extents where file_id = 13;
		

   no rows selected
		

So it looks empty after all; all blocks should be free space.  A closer look:

   SQL> select * from (select file_id, block_id, blocks, block_id - lag(block_id+blocks) over (order by block_id) as gap		
		
        from sys.dba_free_space where file_id=13) where rownum<9;		
	
		
      FILE_ID   BLOCK_ID     BLOCKS        GAP		
		
   ---------- ---------- ---------- ----------		
		
           13          5          4		
		
           13          9          4          0		
		
           13         13          4          0		
		
           13         17          4          0		
		
           13         21          4          0		
		
           13         25          4          0		
		
           13         29          4          0		
		
           13         33          4          0	
	
		
   8 rows selected.
		
   SQL> select * from (select block_id, blocks, block_id+blocks as end, block_id - lag(block_id+blocks) over (order by block_id) as gap