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).