Two words: provisional ballots.
After the disaster of 2000, Congress adopted new measures to protect voters. If someone’s eligibility to vote is questioned because of say, a change in address or immigration status, they are guaranteed the right to cast a provisional ballet. If that voter is later determined to be eligible, their ballot is counted.
If the election is ends up being closer than expected, the delay will involve evaluating these ballots: a process that took two weeks Missouri in 2008, but did not affect the ultimate outcome of Barack Obama’s victory. Provisional votes tend to favor Democratic candidates, so if the election is not decided tonight, it is more than likely that Hillary Clinton will win.
Read more in the Washington Post.