The safety word is an absolute must in any sexual game. When the game being played is somewhat violent or involves pain and potential injury, this is obvious. But even if you're just tying someone to the bedposts with flimsy silk scarves, they might get claustrophobic and panic - this has happened to me once, when I guy I was playing with was having serious trouble with his eyes being tied shut with a scarf, and he had a dreadful time... I still feel really guilty about that one.