You probably know that Comcast is hitting subscribers with overage charges of $10 when they exceed their 300GB monthly data caps. But can customers trust Comcast to measure Internet usage accurately? The nation’s largest cable company points to research it commissioned showing that its data metering is usually accurate, but one customer who contacted Ars was able to prove that he was being incorrectly accused of using excessive data.

Oleg, a programmer from Tennessee who prefers that we not publish his last name, said he got repeated warnings from Comcast that he was using too much data. But the traffic logs from his router showed that “I was not even close to Comcast’s cap,” he wrote. Oleg described his saga in a Pastebin posting , a YouTube video , and in e-mails to Ars.

Oleg received warnings in September and another in October, the latter while he was overseas for a multiple-week vacation with his wife. When they returned home on November 9th, Comcast’s data meter was “showing I used 120 gigs of data, like, while I was gone,” he wrote. Customers can check their usage on Comcast’s website.

Calls with Comcast customer service agents didn’t clear up the problem. "I called Comcast... and was patronizingly informed that 'it must be somebody stealing your Wi-Fi,'" he wrote. "Possible, but highly unlikely. I’m a software developer, Linux kernel contributor, and I take my home security very seriously."

Comcast allows customers to exceed the cap in three months before they are charged overage fees. But Comcast's incorrect metering caused Oleg's courtesy months to be used up in September, October, and November, he said. The waived charges would have totaled $60 across the three months, and he was on the verge of being charged for real. Oleg got to work trying to prove that Comcast’s metering was inaccurate, and he says he disconnected his cable modem from his home network for six and a half days, relying on an unlimited cellular data plan and tethering for Internet access during that time. During that week, Comcast’s meter claims he used 66GB of data, he wrote.

Comcast admits: We screwed up

Oleg provided us his full name and address so we could check into his situation with Comcast. The company investigated the problem after being contacted by Ars and confirmed that its meter readings were inaccurate. “We have reached out and resolved this,” a Comcast spokesperson told Ars. “There was a technical error associated with his account, which we have since corrected.”

During the week he relied on cellular Internet, Oleg said says he used 8GB of mobile data. “That’s the ‘real’ number, as I tried not to vary the normal about of data I normally consume,” he wrote. “I am not a gamer, and I do not stream anything beyond the occasional YouTube video. In short, I’m not a high-bandwidth user.”

Comcast told Oleg that its system had him confused with another customer, he said. “It turns out their system had my modem MAC address entered incorrectly, there was an off-by-one typo that was hard to see so they were counting data from some modem who knows where,” Oleg told Ars.

Before discovering that mistake, a Comcast customer service rep had told Oleg that the company's meter is "94.6 percent accurate." After the truth was revealed, Comcast restored his three courtesy months. Oleg says he would switch broadband providers but Comcast is the only viable option where he lives.

No guarantee that Comcast accurately measures data use

Comcast pays an analysis company called NetForecast to report annually on the accuracy of its Internet usage meters. The latest report released in June of this year said Comcast meters had an average error rate of +0.4 percent to -4.0 percent, meaning that the meters are more likely to undercount data than overcount.

NetForecast tested at 55 homes from January 2014 to April 2015, comparing its own measurements to Comcast's. The report says that Cable Modem Termination Systems (CMTS) in Comcast facilities count the downstream and upstream traffic for each subscriber's cable modem, with each modem being identified by its MAC address.

But there’s no guarantee that Comcast is accurately measuring every customer’s data usage, as Oleg discovered. Customers with less technical expertise than Oleg may not know how to challenge erroneous measurements or even suspect that they’re incorrect. The routers customers typically connect to their cable modems don’t automatically monitor data usage, so customers have to trust that their ISP is accurately recording Internet usage unless they do some extra legwork. “The good news is that you can install a third-party router firmware like DD-WRT or OpenWRT and use bandwidth-monitoring software on it, getting a complete picture of your bandwidth usage,” a How-To Geek article explains.

Comcast has been steadily introducing data caps into parts of its territory, testing customers' responses before a potential nationwide rollout. The caps differ by city and town but generally provide 300GB per month, with customers being charged another $10 for every additional 50GB used. Customers can pay $30 to $35 extra per month for unlimited data in some areas. Comcast CEO Brian Roberts has compared the caps and overage charges to buying gas or electricity, saying that the more you use, the more you pay. But customers aren’t charged less when they use less data, unless they sign up for a “Flexible Data Option” that limits them to 5GB per month and has far more punitive overage charges than the standard plan.

Oleg said he filed a complaint with the Federal Communications Commission, but there’s no rule that would specifically limit or prevent Comcast’s data caps. The FCC could evaluate whether the caps discourage competition from online video services that offer alternatives to Comcast’s cable TV, but it hasn’t made any move to do so.

Oleg suspects he isn’t the only Comcast customer victimized by a MAC address error. His problem appears to be resolved for now, but not without a lot of aggravation. One customer service representative suggested to Oleg that he “just pay 35 dollars a month extra [for unlimited data] and the cap would be gone,” he wrote.

Now that Comcast’s data caps are expanding to more cities and towns, Oleg predicts that “the inaccuracy of their measurements will start to have real material effect on people."