For months I've been warnings investors about the dangers of investing in Citigroup (C). The shares have been rated a Sell on my PortfolioGrader Pro stock-rating website for over a year. Now the stock is in plunging and the company wants the government to reinstate the short-selling ban. The New York Times reports:

The bank has posted four consecutive quarters of losses, caused by billions in write-downs. Nine of its investment funds have cratered this year. And now the bank could face a tsunami of new losses in its once-lucrative consumer loan business as the global economy weakens.

Within the bank's Manhattan offices, television screens have stopped displaying the company's stock price. Traders have begun making jokes comparing Citigroup to the Titanic.

It gets worse. Now that the stock is below $5 a share, many institutional investors can't own it, and that will prompt more selling.

Money managers don't necessarily have to sell Citi immediately. But they would have to get out before the end of the quarter if the stock doesn't recover and may opt to do so now to mitigate potential losses.

"They've got five, six weeks to make decision on whether they're going to get out," Malcolm says. "There's still a lot of institutional ownership of Citigroup. That could change quickly if they have to be out at the end of the year."