Citigroup and other large banks are rife with opaque, off-balance-sheet assets, but indecipherable balance sheets make it nearly impossible for investors to know just how scary this bogeyman is.

Poorly disclosed and little understood liabilities like the ones that helped spark Citigroup's 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. 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.

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.

For more rate offers in your area, check out and enter your ZIP code to find out the best info for your area.

—For more ways to save, spend, invest and borrow, visit MainStreet.com.