From the dos commandline (assuming you are on an smb mounted directory):
  # echo 12345678990 > testfile
  # echo 1234 > testfile
  # type testfile

You now notice that testfile is _not_ containing '1234' as expected.

Systems used:
  - PCclient on PC-DOS 6.2
  - Server: Sarge stable with self compiled samba-3.0.20rc2 still
            running a 2.4.20-686 woody kernel.


The patch:
Index: source/smbd/reply.c
===================================================================
--- source/smbd/reply.c.orig
+++ source/smbd/reply.c
@@ -1610,7 +1610,7 @@
                create_disposition = FILE_CREATE;
        } else {
                /* Create if file doesn't exist, truncate if it does. */
-               create_disposition = FILE_OPEN_IF;
+               create_disposition = FILE_OVERWRITE_IF;
        }

        /* Open file using ntcreate. */