The S&P 500 index suffered its last correction of 10 percent or more in 2011. In fact, it plummeted 20 percent in the July-August period that year.

And Simon Maierhofer, writing on TheStreet.com, finds troubling parallels now in 2015.

• Both years are pre-presidential-election years.
• In the first 5 1/2 months of each year, the S&P 500 index stagnated in a narrow trading range. In both years, the index was little changed on June 12 from the beginning of the year. "What does the S&P have to show for it? A meager 1.71 percent gain," Maierhofer notes.
• When the S&P 500 reached a record high in late April, the percentage of S&P 500 stocks trading above their 50-day simple moving average fell short of its February peak.

To be sure, "although summer might turn into a tough spot for stocks, the damage is likely to be limited and temporary," Maierhofer maintains.

But with the S&P 500 index standing just 1 percent from its record high, many experts are more worried than him.

Greece's debt debacle and signs of weakness in housing are two major factors behind the concern.

"It’s anyone’s bet if the fear in the U.S. is a result of the latest fears about a Greek default or more general concerns that U.S. stock valuations are stretching credulity," writes John Kimelman of Barron's. "It could be a combination of the two, with perhaps a few other reasons thrown in."

When it comes to valuations, the S&P 500 carried a trailing price-earnings ratio of 21.47 last Friday, up from 19.06 a year earlier, according to Birinyi Associates.

As for housing, money manager Barry Ritholtz questions the sector's strength. "We are beginning to see signs that household formation is turning around," Ritholtz writes on Bloomberg. "If that holds up, expect homeownership rates to stop falling, and perhaps even reverse.”

The homeownership rate has sunk to 63.7 percent, a 26-year low.

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