Hang on tight because this game is about to get a lot more interesting!

Stocks skidded for a third session Thursday, even though weekly U.S. jobless claims came in at a four-year low and the Conference Board’s index of leading economic indicators for February spurted a greater-than-expected 0.7%.

In other words, the domestic news seems pretty much hunky-dory. So what prompted the selling?

Well, it’s that old nemesis again, Europe — with a dash of worry about China to boot.

On Thursday, a poll of eurozone purchasing managers (Markit’s flash estimate) reported that factory activity fell to 48.7 in March, from 49.3 in February. A similar survey for China showed a drop to 48.1 from 49.7.

In both cases, a reading below 50 indicates that more companies are seeing contraction than expansion. For Europe, the Markit poll tells us that the economic slump is by no means over.  For China, the survey depicts an ongoing slowdown.

Maybe the U.S. can continue to blow past the rest of the world’s troubles, and the stock market will just zoom higher and higher. However, we’re fast approaching the market’s traditionally weak season (“sell in May and go away”). Meanwhile, among the technical gauges I follow, yellow lights are beginning to flash.

On Tuesday, I mentioned the paucity of individual stocks touching new 52-week highs. Another concern: heavy insider selling.

According to the folks at Vickers, officers and directors of America’s publicly listed companies have executed more than five sell transactions for every purchase over the past eight weeks.

That number could still go a few tenths of a point higher. But it’s getting uncomfortably close to the 5.7-5.8 region that signaled the important market tops in 2007 and 2011.

The bottom line: Use an extra measure of caution with any new stock purchases. It’s also O.K. to do some selling.