On Friday afternoon, the campaign for House Majority Leader Eric Cantor released an internal poll showing the majority leader up 34 points on tea party challenger Dave Brat. On Tuesday, Brat beat Cantor handily. It shocked the entire political world. Journalists on Twitter freaked out:

just got (and sent) several emails that were just exclamation points. — Kate Nocera (@KateNocera) June 10, 2014

I literally can't think of a bigger upset in my time covering politics. — Chris Cillizza (@TheFix) June 11, 2014

I know I'm tweeting the same thing basically over and over again, but just. wow. — Alexis Levinson (@alexis_levinson) June 10, 2014

You can find similar tweets from just about every political reporter in D.C. In fact, Fox News, MSNBC, and CNN weren’t even covering the primary at the time, assuming that nothing newsworthy would happen. But a few minutes after 8 p.m., the Associated Press called the race in favor of Brat. Cantor gave his concession speech not long after. But one question remained on everyone’s mind: How did this happen and how did no one see it coming?

It’s impossible to know the reliability of the internal poll showing Cantor with a 34-point lead. The campaign could have exaggerated it to create a favorable news cycle for the majority leader. But the Washington Post’s Robert Costa reported on Twitter that Cantor’s friends said the majority leader had been told that he was up 20-30 points. The Post’s Chris Cilliza tweeted that GOP strategists were blaming Cantor’s consultants. Clearly, something went wrong with Cantor’s polling team.

But it also says something more about the limits of data. The only other recent public poll from this race came from the Daily Caller and was conducted by Vox Populi. It found Cantor up 52-40 with a margin of error of 9 points. Even if Cantor’s polling team was wildly off, the Daily Caller poll showed Cantor with a solid advantage. In most races, a 12-point lead would be a big advantage. In Cantor’s race, it was surprisingly small, but it still indicated a relatively easy victory for the majority leader. With the limited knowledge we had, Cantor seemed like he was in a good position.

But the key word there is “limited.” Like most congressional primaries, we didn’t know much about this race. Independent pollsters generally stay away from these races. District-level polling is just very hard to do accurately. In other words, data can only tell you so much. When you can’t survey a large enough sample size to represent a given population, a poll isn’t going to be very useful. You may get the outcome correct sometimes, but other times, you’ll be widely inaccurate. That’s not helpful for a candidate.