You took my lock, without my permission, to secure a hazard that you were responsible for; I later came and took my lock, leaving the hazard exposed. Someone then came along and got hurt; who is responsible you or I?
Surprisingly, the Gemara writes that you are responsible; because being that the lock was not yours, you should have assumed that the owner will take it back; thus it is as if you never protected the hazard at all.
Do you agree with the logic? (Talmud, Baba Kama 30a)