Now, it appears that Comcast understands that a few people will go over their limits. "If you choose to use more than 1 TB in a month, we will automatically add blocks of 50 GB to your account for an additional fee of $10 each," reads the FAQ. Comcast also says it will give you two free grace period months in the first year to get accustomed to 1TB.
Comcast says 99 percent of its subscribers do not use 1TB but that is sure to change in the future with the advent of 4K streaming.
Source:
Comcast