The "Hindenburg Omen," an obscure indicator that pops up every few years and gets Wall Street traders chattering about a possible market crash, was triggered this week.
To be sure, the Hindenburg has missed the mark more times than it's been correct, but some analysts are concerned that the omen, when paired with other indicators flashing sell right now, could portend that at least a pullback is ahead this time.
Thursday's session saw the Hindenburg signal trigger at both the New York Stock Exchange and the Nasdaq, marking just the 10th time that's ever happened, according to Roberto Friedlander, head of energy trading at Seaport Global.
There are three criteria for the Hindenburg signal to trigger, Friedlander said:
On Thursday, the number of stocks hitting either 52-week highs and lows on both exchanges totaled more than 2.8 percent, Friedlander said. He added that the McClellan oscillator was negative for both the NYSE and the Nasdaq on Thursday.
And the and the Nasdaq 100 are both above their 50-day moving averages, confirming the Hindenburg at the NYSE and Nasdaq, respectively.
"This is the first time since 2014 that a signal triggered on both exchanges on the same day – now that turns out to not be good for the market," Friedlander said in a note to clients Friday. "When Hindenburg signals flashed on both Nasdaq AND the NYSE (nine times prior to yesterday), mean return 1 month out was a loss of more than 5 percent and a year later [a] mean loss of almost 8 percent."
The indicator has successfully predicted a meaningful pullback less than 30 percent of the time, according to The Wall Street Journal. It did, however, foreshadow a market crash in 1987 and during the 2008 financial crisis.
But Chris Kimble, founder of Kimble Charting Solutions, said there are also signs of deterioration in other parts of the market.
"At the same time this took place this week, the all important banking index, could be completing the right shoulder at (4), of a head and shoulders topping pattern," he told CNBC via email.
Check out this chart of the KBW Nasdaq Bank Index:
Banks have been some of the best performers since the November, as President Donald Trump's election brought hope to investors that key regulations — namely Dodd-Frank — would be scrapped or lessened.
Their deterioration coupled with the Hindenburg has Kimble worried.
Watch: Concerns about the Hindenburg Omen in 2013