If markets pull back 35% or more you could see a prohibition on the short sale of stock, except to perform bonafide market making operations. This was first instituted after the stock market was closed for a week after the 9/11/01 terrorist attacks and trading resumed on September 17, 2001. Nobody could sell short stock on 9/17/2001.
The government also prohibited short sales of 1000 financial stocks for 30 days from Sept 2008 to October 2008. The net effect was:
"The Securities and Exchange Commission's ban on short selling, which sparked a series of similar bans around the globe, was intended to be a "time out" and "restore equilibrium to markets." By the time it expired Wednesday night, the general view was that it added to market confusion and didn't do much to halt the slide in financial stocks. " - WSJ 10/9/08
In 2007 the DOW hit an all time high of 13,264. The DOW lost 52% of its value and hit a low of 6,459 in March 2009.
For comparison sake, the QQQ would fall from $234 to $112 and the DOW falls from 29,568 to 14,488.