I just ran into this reading an old 4bsd tar archive:
if the size on the tape of a link is non-zero, tar x and tar t
will get confused about position on the tape and typically
blow up with a checksum error.  (unix tar evidently had the same bug;
I tried a late research unix tar and it blew up the same way.)

The fix is one line; I added some comments and a little paranoia
against similar future problems.