Trader on the floor of the New York Stock Exchange. Lucas Jackson | Reuters

Stock market selling accelerated Wednesday as major indexes broke through some key technical levels, and a worrisome pattern formed in the chart. The so-called head and shoulders pattern formed when the S&P 500 broke support at 2,800, and from there, analysts said it could lose another 5.4%. The S&P also fell below its 200-day moving average at 2,776, an important level of support and momentum indicator. The index recovered some ground and closed above that level at 2,783, a decline of 0.7% on the session.

"If the 2800 area breaks, it would confirm a head and shoulders top that suggests deeper downside risk to 2650, which is also the 50% retracement of the December to May rally," Bank of America Merrill Lynch technical analysts wrote in a note. The top of the pattern would be the top of the head at 2,950, and 2,800 was considered the neckline.

Head and shoulders chart in the S&P 500

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Source: T3Live.com Scott Redler, partner at T3Live.com, follows short-term technical trends and said the 200-day level is important for the S&P to hold, and he too sees 2,650 as a level indicated by the head and shoulders pattern. The Nasdaq also fell below its 200-day Wednesday, when it dropped through 7,527. It later regained the 200-day, closing at 7,547. The Dow, Russell 2000 and Dow transports have previously broken the 200-day, which is basically an average created from the past 200 days' closing levels. The head and shoulders pattern, however, may be a warning of more weakness and marked a near-term top for the S&P at 2,950. "Actually, the market was weak enough to break the neckline. In the past several years, the neckline has been strong enough and became a place to buy," said Redler. "We've been in a bull market for 10 years, and when the market set up a head and shoulders pattern, it was mostly a better place to buy. This is one of the few times the market set up a head and shoulders pattern, and then traded down another 30 handles."