My current partner and I have a safe word for reasons similar to menthodelic's. In the past, in an abusive relationship, I've tried to set one up because he would make excuses for ignoring "no" and "don't", saying things like "how do I know you're not going to follow it with 'don't stop'?" and basically trying to make me feel like it was my fault for being unclear. He couldn't do that with a random word like "raspberry" or "teapot".