The enthusiastic retail traders who have sent GameStop’s stock up a dizzying 1,635% year-to-date — along with other heavily shorted shares of struggling companies — could inflict even more pain on a Wall Street community still very bullish on the broader market.

Think along the lines of more forced selling by institutions of long-prized positions to cover short bets that are now spiking in value (and losing money in the process).

“Funds in their coverage sold long positions and covered shorts in every sector [last week]. Despite this active deleveraging hedge fund net and gross exposures on a mark-to-market basis both remain close to the highest levels on record (see chart below), indicating ongoing risk of positioning-driven sell-offs,” warns Goldman Sachs chief U.S. equity strategist David Kostin.

Last week represented the largest active hedge fund de-grossing since February 2009, adds Kostin.

Indications of forced selling dotted the stock market during last week’s frenzied activity.

The Dow Jones Industrial Average plunged 620 points on Friday. At 29,982.62 at the sound of Friday’s closing bell, it marked the first time the Dow finished below 30,000 since Dec. 14. All 10 sectors in the S&P 500 closed Friday’s session in the red. The Nasdaq Composite dropped 2% on Friday.

On the week, all three major indexes lost more than 3% for their worst trading performance since October 2020. Some of Wall Street’s most owned stocks — the tech focused FAANG (Facebook, Apple, Amazon, Netflix, and Alphabet’s Google) companies — all notched losses on the week, led by a 7.9% drop in Apple. Again, all indications of forced selling by institutions.

The story was starkly different for traders in some of the most shorted names in the market.

Shares of movie theater operator AMC shot up 54% Friday, bringing their five-session gain to a staggering 180%. GameStop (which has become the face of the mania) shares popped 68% higher Friday. In five sessions, GameStop’s stock has gained 248%. Speaker maker Koss Corporation has seen its stock skyrocket more than 513% in five sessions, helped by a bewildering 52% pop Friday.

The gains are starting to pile up for those traders dipping in and out of Reddit to devise strategies to crush the shorts.

A basket containing the 50 Russell 3000 stocks with market caps above $1 billion and the largest short interest as a share of float has rallied by 98% in the last three months, Goldman’s data shows. This surpassed the 77% return of highly-shorted stocks during the second quarter of 2020, a 56% rally in mid-2009, and two 72% rallies during the Tech Bubble in 1999 and 2000.