538 was widely attacked / mocked because they gave him such high numbers. Many people were surprised by the outcome.

Also, it was clear on election night that neither Hillary nor Trump had prepared a speech for a Trump win. (Watch Trump's victory speech from election night. Hillary had to awkwardly postpone hers till morning because she didn't book the venue long enough to handle a close race.)

I suspected 538's projections were spot on because of math and stuff, but many (most?) people did not.