Slave: "Retrograde ejaculation! Retrograde ejaculation!"
Mistress: "oh that's your safeword! You want me to stop"
Slave: "No, just wanted to let you know I just had one"

lol ... Anyway, more seriously, I've always felt that unless we're going with some well-known protocol -- e.g., stoplight -- it's the sub who should set the safeword. It's something that he has to remember, potentially under extreme stress and pain. I always question when a domme I'm seeing for the first time tells me what my safeword is going to be. HTF am I going to remember "stromboli" or whatever, under the stress of all this