Two different roles. It is the subs' role to warn, the dom/mes' to act on the warning.

I've always said the safeword is mostly for the dominant, so he/she can be assured all is still well and can reign freely over the submissive.

It is a co-owned responsibility to ensure all scenes allow for the successful use of a safe word or sign... and a prior discussion should have occurred to allow for contingencies when a sub enters sub-space.