That miscall is a strike against surveys' accuracy, but it doesn't come as a total surprise to most pollsters and political forecasters, who issued their final predictions under a minor avalanche of caveats and hedging.
"[T]he uncertainty associated with forecasting tonight’s Iowa Republican caucus is about as high as it gets in a major American election," FiveThirtyEight's Nate Silver wrote before voting started Monday afternoon.
In the runup to the caucus, many analysts predicted that lower turnout would favor Cruz, while an influx of new voters would augur a Trump win. But instead, Monday's voting brought both record turnout and a victory for Cruz.
Part of the uncertainty inherent in the caucuses is due to voters' propensity to make up their mind last-minute. In 2012, Rick Santorum won despite never having led in a poll.
A similar last-minute shift could help to explain the better-than-expected numbers for Sen. Marco Rubio (R-Fla.). Rubio's numbers ticked up modestly in the final pre-caucus polls of the state, suggesting the start of a rising trend. Preliminary exit polls show Cruz and Rubio significantly outperforming Trump among the 45 percent of voters who decided sometime in the last week.
Cruz also benefited from superior campaign organization. Cruz also won nearly a third of those who said they'd been personally contacted by a get-out-the-vote effort, a larger share than any other candidate.
"Cruz ground game looks like it came through big time," Marist pollster Lee Miringoff said in an email, noting that Trump's decisions to skip the final Republican debate may have backfired.
Pollsters may have erred beyond just missing a late-breaking shift. Figuring out whose voters will actually show up is always a challenge for pollsters. That's especially true in races like the Iowa caucus, where interest varies wildly from year to year and only a sliver of eligible voters actually make it out.
This year, surveys of the Republican race overstated Trump's ability to turn out voters. While Trump voters were deeply certain of their support, many were only loosely attached to their party, and unfamiliar with caucusing. Compared to Cruz supporters, Trump voters were less likely to have previous caucus experience and more likely to say they planned to show up alone, according to the pollsters at Monmouth University, making them a weaker bet to actually turn out on Election Day.
The record turnout included occasional voters who supported Trump's rivals, but not enough of the new voters he needed to win, Monmouth University pollster Patrick Murray said in an email.
"Cruz and Rubio actually turned out a lot of 'lapsed' caucus goers who only show up sporadically," Murray said. "That was the key."
Cruz's underlying strengths, meanwhile, shone through on nearly every measure but the horse race, according to the final Des Moines Register/Bloomberg Politics poll. Cruz was considered more knowledgeable, more internationally respected and more likable, and won in a one-on-one matchup against Trump.
“The drill-down shows, if anything, stronger alignment with Cruz than Trump, except for the horse race,” pollster J. Ann Selzer told The Des Moines Register when the survey was released.
Turns out, that went for the horse race too.