NEW YORK - When companies are forced to correct their accounting, they like to portray these financial restatements as a cleansing, isolated affair that sets the record straight.

But while most companies don’t make a habit of revising their past earnings reports, one visit to the accounting confessional can lead to an encore appearance, making it best to view restatements with the suspicious eye of an exterminator: Where there’s one, there may be more.

In just the past few weeks, investors have been forced to reckon with announcements that there will be more restatements to come at communications equipment maker Nortel Networks Corp., mortgage lender Fannie Mae and insurer American International Group Inc., despite earlier management assertions to the contrary.

And in January, just before its acquisition by Verizon Communications Inc. was completed, MCI announced one last restatement. The mistake was minor, but it served as a fitting coda to the independent life of a company that restated billions of dollars in accounting entries from the days when it was known as WorldCom.

These are extreme examples no doubt, given the scandals at all four companies. But serial restaters are hardly rare.

Of the nearly 2,000 public U.S. companies that restated past financial reports from 2003 through 2005, roughly 300 were repeat offenders, or about 15 percent, according to Glass Lewis & Co., an advisor to institutional investors. There were 72 companies that restated their results twice in 2005 alone. And during the three-year period, 42 issued restatements three separate times, while six companies revised their filings on four occasions.

The takeaway message here is that even if accounting revisions are not prompted by revelations of outright fraud, they often point to systemic problems undermining the reliability of a company’s financial statements.

Even a simple mistake can be the product of loose internal procedures, flawed oversight or lax external audits.

Ultimately, whether the inaccuracies result from honest error, an aggressive corporate mentality or intentional deception, the credibility of a company’s financial documents becomes questionable.

This is particularly true now that companies are being forced to comb through their books more closely under new federal rules for monitoring internal procedures and reporting on them to the public.

Those reforms, mandated by the Sarbanes-Oxley Act of 2002, have already fueled a disturbing jump in the pace of accounting revisions, with restatements doubling last year compared with 2004’s level. Glass Lewis found that more than 50 percent of last year’s revisions were made by companies that disclosed material weakness with the internal controls that are supposed to ensure the accuracy of their financial reports.

All these troubling signs stand in sharp relief to the reassuring remarks by management that invariably accompany the disclosure of a restatement.

AIG restated its results twice during 2005, revising several years of financial reports in the process. In November, a day after the second restatement, the insurer’s chief executive confidently asserted that he didn’t expect any further revisions. Right now we are not aware of any other issues that require restatement, Martin J. Sullivan said in a conference call with analysts.

Fast-forward four months, AIG has now filed its third restatement of earnings back to 2000 and warned that further restatements may be necessary because the company had not yet completed remediation work in several areas, including derivatives accounting.

Nortel, which restated several years of results in late 2003 and then again in early 2005, revealed earlier this month that it would need to revise past reports yet again. The second restatement came after then-CEO William Owens proclaimed that, financial accountability and trust and transparency will become our watchwords.

This time around, Nortel CEO Mike Zafirovski downplayed the significance of the required restatement, which involves hundreds of millions of dollars in revenue the company reported prematurely, stressing a distinction from the scandal-induced accounting revisions of the past: This revenue is real - it was recognized in the wrong periods, he said in statement. The restatements do not affect the company’s cash position.

While those assertions may be factually true, this multiyear saga at the Canadian company demonstrates how hard it can be for a major corporation to fix a faulty internal system.

A similar tale is playing out at Fannie Mae. The mortgage lender first disclosed accounting-rule violations in September 2004, then warned of further impending revisions last November. Earlier this month it revealed the need for more in yet a third area of the company’s books.

For investors, the lesson is to be leery when companies dismiss their accounting troubles as history - just numbers on a page being shuffled from one period to another with no cash impact. Once a company starts foraging through its books, there’s no telling where the restatements will end.