Calling it an isolated incident, Comcast said a recent move to a new software platform for its broadband data usage meter caused the company to erroneously charge about 2,000 customers for exceeding its monthly residential data plan.

Comcast, which has about 25.63 million residential high-speed Internet customers, said it became aware of the software error from social media chatter and quickly moved to fix the "technical issue" and proactively credit the affected accounts, plus an additional $50 credit.

Ars Technica first reported on the software bug, noting that it resulted in inaccurate readings for two months for roughly 2,000 Comcast broadband customers. Comcast declined to go into great detail with Light Reading about what caused the error, but said it changed how the system was measuring traffic for a small number of customers, causing some customers to see data usage spikes and others to see lower usage. Comcast said its investigation is ongoing.

Ars Technica said the edge case stemmed from the interactions between the updated meter software and the company's new billing system. Among affected customers, their data meters were continuing to collect accurate data, but the numbers being reported to the new billing system were incorrect, Ars Technica said, noting that the problem was resolved when Comcast rolled back to a prior version of its billing software on October 2.

"While updating our data usage meter to a new system, a software error occurred resulting in a small number of our customers being billed incorrectly," Comcast said in its statement to Ars Technica. "We're very sorry for inconveniencing our customers and here's what we're doing to address it: We fixed the technical issue, we're proactively crediting the accounts affected, and we're giving those customers an additional $50 credit to make it right."

Although the glitch effected a small number of customers, it of course raises questions about whether ISPs or third parties should be responsible for governing data meters that form the basis of usage-based billing policies for broadband services.

Comcast said it continues to use NetForecast as an independent, third-party firm to audit the accuracy of its meter. According to NetForecast's abstract report for Comcast from 2018, based on more than 4 million audit measurements, Comcast's meter met the company's accuracy goal at month's end for 94.5% of the months studied, with undercounting occurring in 4.6% of those months. Comcast's stated goal is for the meter to be accurate within +/-1%. NetForecast's report also showed that Comcast's meter system "consistently improved" in 2018.

Figure 1: Source: NetForecast.

Comcast stressed that consumer trust in its data meters is critical and resolving this technical issue is a top priority for the company, noting that it proactively moved to reverse any incorrect charges and provide the additional credit.

In the majority of its US markets, Comcast currently limits monthly data usage on residential cable broadband service to 1 Terabyte. Once that ceiling is exceeded in a given month (and after two courtesy months), the operator charges an additional $10 for each additional block of 50 Gigabytes, with overage charges not to exceed $200 each month. Comcast also provides an unlimited data option for an additional $50 per month. With its "Flexible Data Option," tailored for lighter Internet users on lower speed tiers, Comcast credits $5 if the customer's monthly usage doesn't exceed 5 GB, and charges $1 for each GB of data used beyond that 5 GB monthly limit.

Comcast confirmed that it currently uses a broadband usage data policy in its West and Central divisions, but does not currently enforce the policy in its Northeast division. Comcast's data plan doesn't apply to residential customers on its FTTP-based Gigabit Pro service (2 Gbit/s symmetrical), its prepaid Internet service or its business service customers.

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— Jeff Baumgartner, Senior Editor, Light Reading