IPhone 5 May Burn Through Excessive Cell Data on Verizon - NYTimes.com:
Over the weekend, some iPhone 5 users complained to Glenn Fleishman, a writer for TidBITS, an Apple news and troubleshooting site, about their phones’ mysteriously using Internet data over the cellular network for no clear reason.

Verizon Wireless has acknowledged the problem, and says it has to do with the phone’s use of cellular data even when it is connected to Wi-Fi. A solution is being issued to Verizon customers, and they will not be charged for the erroneous data use, the company said.

I haven't found a good description of when or why this is happening, but it seems an obvious enough issue that you wonder who is testing this stuff.