תחילתו בפשיעה וסופו באונס

The Gemara relates: There was a certain man who deposited money with another, and the bailee placed it in a willow hut from which the money was stolen. Rav Yosef said: Although with regard to thieves, placing the money in the hut is effective safeguarding, with regard to fire it is negligence, as it is likely to burn. Therefore, it is a case where the incident was initially through negligence and ultimately by accident, and the bailee is liable to pay. And some say: Although with regard to fire it is negligence, with regard to thieves it is effective safeguarding. Therefore, it is a case where the beginning of the incident was negligence and ultimately the damage was caused by accident, and the bailee is exempt. The Gemara concludes: And the halakha is: In a case where the incident was initially through negligence and ultimately by accident, the bailee is liable to pay.

it begins with negligence - because what if a flame falls on it and it's discovered that he's negligent because he didn't bury it

and ends with an unavoidable accident - that it was stolen and theft from a free watcher is considered an unavoidable accident

It is proven like this below (93b) that the Halachah is that he is liable. Abaye asked "if so, the beginning was negligence and the end was Ones. He is liable!".

It is proven like this also in Bava Kama (56a).

This is only when it is possible that the Ones would not have occurred had he not been negligent at the beginning;

However, if he was negligent, and it went out to the swamp and died, he is exempt, for the angel of death [would have killed it no matter where it was].

We must say that the shack of willow branches was a good guarding regarding theft, just like in the ground. Therefore it is called Ones regarding theft.

If the watchman was negligent and did not guard the item properly in one area, even if ultimately the item was misplaced because of an unavoidable accident in another area, he is considered negligent and must pay. An incident occurred where one deposited money with another and the watchman left them in a partition of reeds and were stolen from there. The Rabbis said that although this is a proper guarding with respect to theft, it is not a proper guarding with respect to fire and because he did not hide it in the ground or a wall of a building, he is negligent, and in any case where one was negligent in the beginning but an accident occurred in the end, the watchman is liable. The same applies to anything similar.

Mordechai says in perek hayashan"if he originally made the succah in a place that would be too difficult to sleep, he hasn't fulfilled his mitzvah at all, even for eating. " This is not what "teshvu ke'ein taduru" is to me.