According to the Washington Post, a customer would have to download the equivalent of 1,000 songs or four feature films a day to trigger a disconnection warning, after which Comcast gives customers a month to fix problems or upgrade their service before they are disconnected. The basis for Comcast’s actions appears to be in its “acceptable use policy” for its “unlimited” service which enforces the invisible download limit:
The 23-part policy, states that it is a breach of contract to generate “levels of traffic sufficient to impede others' ability to send or retrieve information.”
Customers argue that "unlimited Internet," as it's advertised, should mean simply that. However, Comcast refuses to even disclose just how much is too much. Comcast says that affected users can upgrade to business accounts, but that option is priced to reflect uptime and service guarantees that most home users can't afford.