1. Stein voters realizing their vote could usher in the end of gay rights, unfettered control of tax and spending policy by selfish (often non-American) billionaires), foreign powers' control of US foreign policy. It would also potentially cause an end of voting rights, but these last are not a concern of Stein's lily white, upper class base.
2. A strong turnout by black, Hispanic voters over the weekend. Would affect almost every undecided state on the map except perhaps New Hampshire.
3. Unexpected turnout by millennials. Don't hold your breath. But this could matter in states like Wisconsin and Colorado. And let's not forget that pot is conveniently on the ballot in nine states this year, including key battlegrounds Arizona, Florida and Nevada. In Arizona and Nevada, the ballot initiatives are for recreational rather than merely medical marijuana: significantly greater in consequence and a bigger draw.
The flip side:
4. Huge Election Day turnout by evangelicals, who have developed a most intriguing affection for the only candidate this cycle who was accused of raping a 13 year old child. This happened in 2004: Democrats made all their turnout targets but didn't anticipate that in the anti-gay marriage petitions in states like Ohio would cause churches all over the state to bus followers to the polls at their own expense.
5. Voter intimidation and successful attempts at vote suppression. Already well documented and a proven success in 2004.
6. The mysterious "hidden Trump vote" suddenly manifests. Racist white voters come out en masse to support the candidate who has reflected their true feelings, feelings they never wanted to disclose to pollsters, including computerized polls.
7. Depressed turnout by anti-Trump voters who never cared for Hillary and like her even less now that they think she might be a criminal.
8. The possibility that Democrats focused so much on the early vote that all their voters turned out, with few left to show up on Election Day itself.
9. Depressed turnout by anti-Clinton voters who can't bring themselves to vote for someone who is under the control of a foreign power, possibly raped a child, and almost certainly slept with other children in cocaine-filled parties. Such voters would have to be less religious, since the US Conference of Catholic Bishops, Ralph Reed, James Dobson, Tony Perkins, Jerry Falwell Jr., Franklin Graham, CBN, and other white Christian identity groups have led their followers to back Trump over Clinton by margins of third to sixty points. If any of these were to occur, potentially all of the 179 electoral votes I've placed in the undecided column could move in one direction, leading to a historic victory (431 electoral votes) for Clinton, or a shockingly broad victory (314 electoral votes) for Trump. But right now, neither extreme outcome seems likely. So lets put the pie-in-the-sky states back in to the appropriate column. This yields a floor of 249 electoral votes for Clinton and 164 for Trump.