The Reddit user skelatwork got the message shown above in the picture for using around 11 GB of data over his cell phone for the month. He will be throttled the entire next month most likely per AT&T’s wording as follows:
AT&T Starts Throttling Heavy Data UsersTweet
Starting October 1, smartphone customers with unlimited data plans may experience reduced speeds once their usage in a billing cycle reaches the level that puts them among the top 5 percent of heaviest data users. These customers can still use unlimited data and their speeds will be restored with the start of the next billing cycle. Before you are affected, we will provide multiple notices, including a grace period.