Norma Butterfield, mother of Slack Technologies CEO Stewart Butterfield, rings a ceremonial bell along with New York Stock Exchange (NYSE) President Stacey Cunningham during the Slack direct listing in New York, U.S. June 20, 2019.

Casey Stengel once said, "Never make predictions, especially about the future." If you're looking to prove that Stengel was right, that humans are terrible about predicting even the near-term future, look no further than what has happened in the IPO market this year.

The first half of 2019 was a big surprise to many IPO investors.

The IPO market was mostly shut down in the first quarter. Going into the second quarter, there was much trepidation that there was too many IPOs coming to market (more than 200 were expected in 2019). Oh sure, there would be demand for Uber and Lyft, but who wanted all the other stuff?

Wrong, wrong, wrong.

In the second quarter, 62 IPOs raised $25 billion, the most active quarter by deal count in four years and the most capital raised in five years, according to Renaissance Capital. The average return was an eye-popping 30%. Technology and biotechs led the charge and activity would have been even higher were it not for below-average activity in the energy and financial sectors.

Kathleen Smith, who tracks IPO at Renaissance Capital and runs the Renaissance Capital IPO ETF, credits a "perfect storm" of positive news for IPO investors: reasonable initial pricing from most companies that was in many cases below their publicly traded peers, and the overall strength of the broad market, which has brought in momentum investors.

"Investors are awarding a high multiple to the stock market this year, so IPO investors are benefiting," she said.

Plant-based foods maker Beyond Meat was the best performer, but there were also strong gains from Pinterest, Zoom Video, Chewy and Slack, which went public using a direct listing.

Notable IPOs 2019

Beyond Meat up 542%

Zoom Video up 140%

CrowdStrike up 89%

Chewy up 50%

Pinterest up 38%

Slack up 39%

And what about the two most-anticipated, supposedly most in-demand deals of the year? Uber's $8.1 billion deal was the largest IPO since Alibaba in 2014 and the largest IPO of a U.S. company since Facebook in 2012. It promptly disappointed, dropping 8% on its first day. It's rival, Lyft, went public at the end of March, and it too disappointed.

IPO disappointments

Uber down 1%

Lyft down 12%

But those were the exception, and have not dampened enthusiasm for a whole new crop slated for the second half of 2019. The IPO pipeline now contains 60 companies looking to raise about $11 billion, roughly half of which have filed or updated in the past 90 days.

The big names include WeWork, Peloton, online food and goods delivery service Postmates, online mattress firm Casper, peer-to-peer fashion e-commerce site Poshmark and, of course, Airbnb.

The question is, how long is this going to go on? At this rate, we could hit the all-time record of $96.9 billion raised in 2000. That, of course, was the internet bubble.

Smith frets that as each deal succeeds, investors will get less disciplined, and the initial first-day pops we have seen so often will become a thing of the past.

Take advantage of it while you can, Smith advises: "Any private company that is not proceeding to go public in this market should have their head examined. The market is wide open, and it's not always going to be."