A notice of suspension and delisting for American International Group Inc. (AIG) was erroneously posted on the Web site of the New York Stock Exchange for three hours Wednesday, a day when shares of the insurer lost more than a fifth of their value following a reverse stock split.

The exchange, part of NYSE Euronext (NYX), took down the notice at 3:31 p.m. EDT after the insurance company brought the matter to its attention. NYSE said in a press release that it regretted the error and that AIG is not subject to suspension and delisting.

The erroneous notice was generated by an automated feed that draws on different status items, such as stock prices. An NYSE spokesman said that the system misread the 1-for-20 reverse stock split that went into effect after AIG shareholders approved the move late Tuesday.

"It generated a flag that indicated the company was up for suspension," the spokesman said.

It's unclear what it was about the reverse stock split that had triggered the notice. During a reverse stock split, the old security is suspended from trading at the same time the new one begins trading.

An AIG spokeswoman said, "Our understanding is that this was an NYSE systems error."

The exchange typically issues delisting or suspension notices when a company's stock falls below $1 a share for an extended period or if its market capitalization drops below $25 million. It eased those rules amid the economic downturn as once-healthy companies saw their share prices plummet.

The errant notice didn't appear to have a significant impact on trading in AIG shares Wednesday. The stock was already down 20% by the time the notice was posted. AIG shares closed down 22% at $18.08, and were little changed in after-hours trading.

Shares of companies that undertake reverse splits often fall as investors remain skeptical about the prospects of such companies.

In addition, some market participants noted that AIG's sharp decline Wednesday could be because it is easier to take short positions in higher-priced stocks. Also, many of those trading in AIG shares were purely speculating and liked having the stock around $1, so those folks likely sold Wednesday.

AIG shares have been a risky bet for some time, losing more than 90% of their value over the past year. The company, which was at the forefront of the financial crisis and is now mostly owned by the U.S. government, has been looking to pare assets and raise capital to pay back the tens of billions of dollars in government aid it received.

-By Jacob Bunge and Lauren Pollock, Dow Jones Newswires; 212-416-2356; jacob.bunge@dowjones.com

(Geoff Rogow and Annelena Loeb contributed to this article)