Wikipedia says the 2018 October surprise was Trump whipping up fear over a migrant caravan. He sent troops to the border even though the Mexican government was blocking them from getting up near the U.S.
I presume DJT will try something this September or October. Whether it will get more publicity than his normal attention-getting is anyone’s guess.
It wasn’t a myth last year in the New Jersey governor’s race. Ok technically it was a last week of September surprise. An ally of the Republican candidate was given an unredacted copy of Mikie Sherrill’s military record in June. The last week of September it was leaked that she was part of the Naval Academy cheating scandal the year she graduated. She was disciplined for not snitching. I think that fits the definition.
Bush the Younger vs. Kerry. Osama Bin fucking Laden releases a public broadcast on October 29.
ETA: oh, this has already been cited, sorry