The closest an October surprise has come to defeating a Republican was the drunk-driving story Al Gore dropped against George Bush the Friday or Saturday before the election.
Bush was up in the polls, but didn’t have any time to respond to the drunk driving story, thus the final vote was closer than it should have been, and we had the Florida debacle.
October surprises are almost moot now that early voting smears “election day” all over October and November. Can’t change a vote if you’ve already cast it.
That’s assuming EVERYONE votes early. Fortunately it’s a pretty small percentage. (But it still gives the rats a better idea how many votes the need to gin up.)