Reference for Ketubot 5:11

אונסא דשכיח שאני דכיון דאיבעי ליה לאתנויי ולא אתני איהו הוא דאפסיד אנפשיה

An accident which is frequent is different, for since he should have stipulated it and he did not stipulate it, he caused himself the loss.

Jastrow