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S&P suffers bizarre late-night swing, more ahead?

Traders are warning that many rocky moves could be ahead in the last trading weeks of the year. This, after stock futures took a massive tick down to a five-week low in late trading before recovering, with no apparent reason behind the drop.

"The market was relatively thin, and somebody came in with enough volume to move the market," said Rich Ilczyszyn of iiTrader. "Stops were triggered, and it kind of snowballed down from there."

The swift move took the S&P 500 E-mini futures, the Dow Jones E-mini futures, and the Russell 2000 mini futures all to five-week lows at 10:08 p.m. EST Sunday. Then, after shaving off as many as 11.5 points—or 0.65 percent—in a minute, the S&P 500 came back by 4 a.m. EST, and opened higher on Monday morning.

Explanations for the move varied.

"The talk is of a large order, but there is no consensus as to whether it was a 'fat finger' error or just an awful execution—or maybe just price manipulation to see if they could force stops below a technical level," Jim Iuorio of TJM Institutional Services wrote in a e-mail.

Whatever the cause, traders say that the 50-day moving average marked the market's bottom.

(Read more: Scared or not, you have to buy stocks: BlackRock's Koesterich)

"I have no clue what that was all about," admitted Brian Stutland of the Stutland Volatility Group, "but I do know that as soon as the futures hit the 50-day moving average, they bounced hard."

The concern now is that more and more of these jerky moves could be ahead, especially on Tuesday and early Wednesday, given that the Federal Reserve's decision on the future of its asset-purchasing program will be released on Wednesday afternoon.

"Coming into a big event like the Fed, especially in the last few weeks of the year, it's going to be a trading affair," Ilczysyzn said. "You're going to see big moves up and big moves down."

(Read more: What every investor has to prepare for this week)

After all, there may simply not be enough volume to support major orders without disruption.

"There's just very little volume out there," Ilczyszyn said. "When I look at the market, I'm seeing that people are in total holiday mode—it's all football and eating and family."

—By CNBC's Alex Rosenberg. Follow him on Twitter: @CNBCAlex.

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