It depends.. first off, the rollover is only for wirless, not wired internet services.
Second, the rollover is a little tricky.
IF you have data left over at the end of one billing cycle to the next, it does in fact, roll over. BUT, you need to use up ALL your data in the next month to use the rolled over credits.
Any left over data that was rolled over and unused, goes away at the end of the next billing cycle.
Keep in mind, if you have used up all your data in a month, even if there is unused rollover data left, NONE will rollover, as you can not roll over unused data a second time.
IF you use up your extra rolled over data, you will be charged, as you said. That extra data does NOT qualify for being rolled over, as it is NOT left over regular minutes.
On the plus side, if you call mobility customer service and plead your case, they MIGHT grant you a credit for the 15 dollars you were charged. No primises, of course, but on the other hand, you also have nothing to lose by trying.
Fair notice: I am another AT&T customer, not an employee, and i'm giving my opinions based on what i've read from other folks posts. I don't speak for AT&T, and could in fact be totaly misinformed. So, keep that in mind when making your choices.
That said, i still think I have the information right, and if not, hope someone will correct where i mistated. Good luck.