Netflix Inc., the top-performing stock in the Standard & Poor's 500 index this year, fell as much as 7.4 percent after saying the introduction of new chip-based credit cards cut off some customers, leading to lower-than-estimated U.S. subscriber growth last quarter.

Profit also fell short in the third quarter as the online video service sped up the recognition of some programming costs. Netflix signed 880,000 new domestic subscribers in the period, according to a statement posted on its website Wednesday. That missed the 1.25 million average of eight analysts' estimates compiled by Bloomberg.

Netflix disappointed on two key metrics, subscriber growth and programming expenses, that investors have used to value the company during its rapid global expansion. Founder and Chief Executive Officer Reed Hastings has been operating Netflix at break even while he races to sign new customers. The accounting change increases programming costs on some of the company's $10.4 billion in long-term TV and film commitments.

"This is a subscriber-driven story so any slowdown, for whatever reason, is a cause for concern for Netflix investors," Paul Sweeney, an analyst with Bloomberg Intelligence, said in an e-mail.

Shares of Netflix fell 6.7 percent to $102.83 at 9:37 a.m. in New York, after declining as low as $102.11 for the biggest drop in 1 1/2 month. The stock, which split 7-for-1 in July, has more than doubled this year.

The introduction of the new credit and debit cards, designed to help prevent fraud, was one of several factors weighing on signups during the quarter, Hastings and Chief Financial Officer David Wells said on a video conference call.

That change interrupted the payment system used by Los Gatos, California-based Netflix and its customers, leading to what Hastings in his letter called an "inability to collect" from customers who hadn't updated their account information. The result was more-than-expected turnover among subscribers, or "involuntary churn."