In 1948, Thomas E. Dewey was running against the incumbent President Harry S. Truman. All the polls had Dewey in the lead and there was no doubt he wouldn't win. The Chicago Daily Tribune was facing a printer's strike and was so sure of a Dewey win that they pre-published this now-famous headline:
That's Truman, the then-winner, holding up what is the biggest mistake in newspaper history.
When I listen to Trump say he's going to win and that the polls have it wrong, I think about that "Tribune" headline. Of course, that was most likely a one-off situation. There are just too many polls with Trump losing by double digits or by at least 6 percentage points. It really doesn't matter if Truman and Trump have, coincidentally, the same first four letters in their names. Which is weird in itself.
Dewey defeats Truman: http://www.chicagotribune.com/news/nationworld/politics/chi-chicagodays-deweydefeats-story-story.html
Clinton Vaults to a Double-Digit Lead, Boosted by Broad Disapproval of Trump (POLL): http://abcnews.go.com/Politics/clinton-vaults-double-digit-lead-boosted-broad-disapproval/story?id=42993821
RealClearPolitics: Polls: General Election: Trump vs. Clinton: http://www.realclearpolitics.com/epolls/2016/president/us/general_election_trump_vs_clinton-5491.html