In reply to GrahamD:
> In sailing the tails would be 'seized' back to the main part to prevent this.
No, in sailing you'd use the knot for its intended purpose. Which is not joining ropes.
In reply to Dave Perry:
> A reef knot, as its name implies was originally used to 'reef sails' - ie reducing the amount of sail on a ship.
Yep, it's for tying up the bunt of the sail (bundling up the surplus sailcloth). It's a knot, not a bend, and should never be used as one.
So, sure, it's safe with the double fisherman's, but it's misleading to think of that as a reef knot with stoppers when it's more properly a double fisherman's with reef knot separator.