The Summerween Trickster's warning is to find 500 pieces of candy before the last jack-o-melon goes out; yet when the Mystery Shack is seen after all the jack-o-melons go out when the kids lose all the candy, Stan's jack-o-melon is still lit.
When Soos hits the Summerween Trickster with his truck, Gormy cannot be seen. Near the end as Soos is eating the Trickster, Gormy comes out.
When the kids go out Trick-or-Treating, it appears to be just after sunset. Dipper checks his watch, and it says 6:00. However, in eastern Oregon (where the show is set) in June, the sun sets at around 9:30 PM.