Poorly disclosed and little understood liabilities like the ones that helped spark

Citigroup's

(C) - Get Report

free fall last week could still threaten the stability of the financial giant and other large banks.

Citigroup's Nov. 19 announcement that it would move about $17 billion in obscure assets housed in off-balance sheet Structured Investment Vehicles (SIVs) back onto its books appears to have played an important role in the bank's near-meltdown last week, leading to the government

bailout

announced Monday.

Citigroup will receive a new, $20 billion preferred equity investment from Treasury's Troubled Assets Relief Program, on top of the $25 billion it received last month. Treasury, the Federal Deposit Insurance Corp. and the

Federal Reserve

will backstop $306 billion of risky loans and securities backed by commercial and residential mortgages as part of the deal. Citi said it has no more exposure to SIVs, and other banks have redoubled their efforts to distance themselves from this now-scary acronym.

Even most sophisticated investors were unfamiliar with SIVs until late last year, when Citigroup suddenly announced it had to bail out several of them, assuming some $70 billion of additional debt in the process.

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But scary as they are, SIVs are merely one type of off-balance sheet obligation. There are lesser-known (for now, at least) instruments like Qualifying Special Purpose Entities (QSPEs) and Variable Interest Entities (VIEs) that investors should know about. There are also easier-to-understand things like untapped credit lines to consumers and businesses. If these obligations -- intended to isolate risk and provide a cheaper avenue for financing -- come onto the balance sheet, banks need to set aside more capital to insure against the risk of default, which can hurt returns to investors. Also, raising such capital in the current environment is expensive and difficult.

Are there other off-balance-sheet ogres out there?

JPMorgan Chase

(JPM) - Get Report

,

Goldman Sachs

(GS) - Get Report

,

Wells Fargo

(WFC) - Get Report

,

Morgan Stanley

(MS) - Get Report

and

Bank of America

(BAC) - Get Report

all make reference to off-balance-sheet obligations in regulatory filings, but disclosures are far from clear, and there is no way to know how thorough they are.

Recent financial statements for Goldman Sachs and JPMorgan paint at least a partial picture of the size and type of off-balance obligations at two of the most highly-regarded of the large banks.

Goldman Sachs had $65 billion in VIE assets as of August 2008, according to a third-quarter regulatory filing. It describes its maximum exposure to loss from those assets as $22 billion. Goldman also has about $2 billion in exposure to QSPEs, according to the filing. A spokesman declines to comment on whether the firm has any additional off-balance-sheet obligations. But a Goldman executive, who declines to be on the record because he is not authorized by the firm to discuss material financial matters not already disclosed in public filings, says it does not.

JPMorgan had about $1.7 trillion in "off-balance-sheet lending-related financial instruments and guarantees" as of Sept. 30, according to its third-quarter earnings report. The largest portion of this, about $910 billion, is unfunded credit card and home equity lines. The bank points out in a footnote that these are unlikely to be tapped all at once and it notes that it can reduce or cancel the credit card and some of the home equity lines.

Discussing these types of obligations with

TheStreet.com

on Nov. 7, Tom Brown, head of financial services hedge fund Second Curve Capital, said that he believed the market understood and was comfortable with the off-balance-sheet exposures of Citigroup and other large banks. In a follow-up call Monday, he admitted he was wrong.

"It's amazing on a $2 trillion balance sheet that the disclosure of a $17 billion off-balance-sheet obligation coming back on the balance sheet would create that much concern, but it did," Brown told

TheStreet.com

on Monday.

Brown notes that because financial institutions like Citi depend so heavily on the confidence of other institutions they trade with and borrow from, merely the

perception

of instability can lead to actual instability.

Whether or not investors were right to be concerned about a mere $17 billion on Citi's large balance sheet, the fact is that in the current environment they are concerned. And there is plenty more for them to fret about on the balance sheet of Citi and other banks.