Case: The plaintiffs (=pl) rented out an apartment, beneath theirs, to the defendants (=def) for six months. They left the apartment and canceled the checks for the final three months before the end of the contract. Pl wants def to pay for that time, as they must respect the contractual obligations. Def complained that while when they rented the apartment the weather was such that there were no problems, during the winter, when they needed to use heaters, the electricity often blew for insufficient wattage. The house was also not reasonably insulated and as a result of the two situations, the family suffered tremendously. After initially ignoring complaints, pl finally called an electrician, who informed def that their usage of electricity would have to be limited. Once when pl went away for Shabbat and guests used it, def’s electricity blew and was out for all of Shabbat even though def claims to have followed the limitations. The two sides agreed to summon the aforementioned electrician for testimony and that each would fully accept his word (like two valid witnesses).

Ruling: The electrician testified that the electrical grids of the two apartments are linked in a way that a short circuit in pl’s apartment (the main one) blows the electricity in def’s apartment (the accessory apartment), but not vice versa. He said that he had to give instructions to both sides to limit use of electricity in order to prevent power outages.
Beit din accepts def’s account that they did not know that they had power outages due to what was happening in pl’s house, which explains why the electricity would go out and then come back without their intervention (pl lifted the fuse). Pl were aware and were generally careful (which their Shabbat guests were not), but the whole setup was problematic. Pl did not claim that def was aware of the problem before renting and, in any case, they need to prove, in a case where there is a clear blemish in the object of the transaction, that the other side knew and relinquished their right to complain.
Beit din determines that it is not reasonable for a renter’s electricity to be at the mercy of the landlord, which can cause several problems, some of which occurred. The fact that def lived there for a while does not preclude their making the claim of mekach taut (a sale based on false premises), as long as they did not continue to use the rented entity after the problem was discovered (Rambam, Mechira 15:3; Shulchan Aruch, Choshen Mishpat 232:3). It is noteworthy that pl was unable to find a renter after def left for quite a while, which is some indication that the unit is more appropriate for a vacation home, as it was originally intended, than for a permanent rental home.
Therefore, def were justified in breaking the contract and need not pay anything further to pl.