Results from the British election show one clear loser: POllsters. Every pollster virtually had the parties in a dead heat. But in the end it was a slaughter for the Torys. Even Nate Silver, favorite of the libs here, was way off, offering an explanation
What We Got Wrong In Our 2015 U.K. General Election Model FiveThirtyEight
But the truth, which no oe has brought out, is that people tend to lie to pollsters because to admit being for the conservatives is to be perceived as "mean." The same thing probably operates here: people claim to like Hillary or whatever Dem but then vote GOP. This will play out on the coming election for sure.
There may be a bit of truth in that in the US, but not much. For example, when asked in telephone polls, 40% of Americans have generally identified themselves as conservatives. However, in exit polls, conservatives are around 35% of the electorate. This means that, if true, only 10%-15% of conservatives, and 1 in 20 Americans, fibbed about being a conservative in the exit poll. That Republicans have lost the popular vote in 5 of the past 6 Presidential elections suggests this isn't an issue in the United States.
The last time I remember watching a British election where the exit polls were wrong was in 1992, when the BBC, based on exit polls, called a majority government for Neil Kinnock when in fact, John Major wound up winning. When the BBC did an analysis after the election, they found that a sizable minority voted for the Tories but then said they voted for Labour when asked by an exit pollster after they left the polling booth.
Polls in the last Canadian election were also wrong. All of the polling firms had the Tories around 34% going into election voting, which probably would have meant a Conservative minority government, but the Tories wound up winning 38% and a majority government.
But people misunderstand polling. Polling isn't infallible. A poll is an attempt to get a snapshot of the electorate at large. It is a probability assessment, not a certainty.
The polls were pretty much spot on during the last two Presidential elections. In fact, in 2012, the polls underestimated Obama's strength. It was conservatives who were wrong in 2012, and embarrassingly so, with all these "skewed polls" and such nonsense. For example, Republicans genuinely believed that they would win Pennsylvania. But in the 45 state polls prior to the election, Obama led in 44 and Romney was tied with Obama in 1. Clear-headed analysis would have suggested the Republicans would have had a very long shot in PA, but they were so blinded by their hatred for Obama that they created all these fantasies that they were going to win.