The is flashing a technical warning sign.
The benchmark index is about to enter a 'death cross,' a technically bearish move where its 50-day moving average crosses below its 200-day moving average.
Matt Maley, equity strategist at Miller Tabak, is not sweating the technical move just yet.
"It is usually a compelling indicator but not always initially," Maley said on CNBC's "Trading Nation " on Thursday. "In 2016, when the death cross took place and the market just kind of fell out of bed in very early 2016. However, in 2011, the death cross was immediately followed by a sharp 7 to 8 percent rally… It's an indicator for later on and we may not get that lower low until we get into the New Year."
Maley is instead watching a different technical move on the S&P 500: its support level at 2640. That was its closing lows in October and November.
"But, I think the more important level is even down at the 2600 level, 2580 to 2600. That's the lows we saw from both February and April and the intraday low from October," said Maley. "That I think is the more important low, that's the line in the sand, and if we break below that level then it's kind of "Katie, bar the door."
The S&P 500 would need to drop by around 2.4 percent to get down to 2600. It has not closed below that level since April.
The unpredictable swings in the S&P 500 has Susquehanna market strategist Stacey Gilbert worried investors aren't prepared for the next big rally or sell-off.
"My concern is that people are not set up for the potential gap risk," Gilbert said on "Trading Nation" on Thursday.
For example, from its Nov. 20 low to Dec. 3 high, the S&P rallied more than 6 percent. Going back to 2010, the average rally over a seven-day period was 3.3 percent, says Gilbert.
"I like owning tails, I continue to like owning tails. Those are owning out-of-the-money options for the potential of those big gap risks," said Gilbert. "That could be both to the upside and the downside, I'm not saying I only want to own downside because as we saw we can move just as quickly to the upside here… I like owning that optionality where you have exposure to extreme type of moves."
Thursday's sell-off erased most of the S&P 500's year-to-date gains. At its September record, the S&P 500 was up 10 percent for the year.