Microsoft Corp. said Friday that it fixed a glitch in its online advertising platform that overcharged some merchants by hundreds, and in some cases thousands, of dollars.

In a statement on one of its blogs, the Redmond-based company said the bug in its adCenter service had pushed up "cost per click" rates -- or what an advertiser pays Microsoft each time a Web surfer clicks on an ad.

Microsoft launched adCenter last year, but still lags Google Inc. and Yahoo Inc. in making money from text ads on search results pages.

The software maker said the affected advertisers' accounts will be credited the amount they were overcharged.

On a message board on the Web site WebMasterWorld, some users wrote that they were overcharged by a dollar or two per click. One member, writing under the name Chrisuk, said he was charged $900 for a single click.

James Krewson, who owns Finderscheapers.com, a price-comparison Web site, said he noticed the problem Thursday night. He had used adCenter to buy keywords related to selling DVD box sets online for 19 cents per click, but saw that he was charged $581.31 for four clicks. He sent an e-mail to customer service, and got a response Friday morning.

"It's the biggest screw-up I've ever seen," said Krewson, who spends about $10,000 per month to advertise on the Web. "I kind of had fun watching it. If I was one of those people who had been charged $12,000, I would not be in the same mood."