Putting aside the question of why Cantor lost

What I want to know is how something like this happens.

An internal poll by House Majority Leader Eric Cantor's (R-VA) re-election campaign had him with a 34-point lead over primary opponent economics professor David Brat.

The poll, conducted by John McLaughlin of McLaughlin & Associates between May 27 and May 28 found Cantor with 62 percent to Brat with 28 percent, according to The Washington Post on Friday.

Cantor lost to Brat on Tuesday. With 90 percent of precincts reporting Brat defeated Cantor 55.4 percent to 44.6 percent. It was perhaps the biggest upset of any primary contest in the 2014 election cycle.

I know of a lot of mistakes in the design and analysis of polls (I've even made a few), but I can't recall any that produced results like this. I've seen numerous articles on what went wrong with Cantor's campaign. I want to know what was going on with his pollster.