Back in September, Nigel Warren rented out his bedroom in the apartment where he lives for $100 a night on Airbnb, the fast-growing Web site for short-term home and apartment stays. His roommate was cool with it, and his guests behaved themselves during their stay in the East Village building where he is a renter.

But when he returned from a three-night trip to Colorado, he heard from his landlord. Special enforcement officers from the city showed up while he was gone, and the landlord received five violations for running afoul of rules related to illegal transient hotels. Added together, the potential fines looked as if they could reach over $40,000.

Mr. Warren, like many if not most Airbnb users, had not read the terms and conditions on Airbnb’s Web site telling him not to break any laws (while also wiping the company’s hands clean of responsibility for hosts’ compliance with those laws).

So he gulped hard, begged his landlord not to evict him and told him that he would attend the mandatory administrative hearing related to the violations and pay any fines. Then, he gulped harder and hired a lawyer for $415 an hour. He also fired off a note to Airbnb, which collects the nightly fee on behalf of its hosts and keeps a bit for itself.