Halloween is the first season 2 episode to have aired on the Boomerang app.
It's about Fred, who after having a bad childhood incident on Halloween involving Baba Yaga, believes a mystery can never be solved on that day. But when she resurfaces in the present, it's time for Fred to solve the mystery once and for all.
It's not as big as a plot hole as the headless snowman being a drilling machine in 'a scooby doo chirstmas' (serriously, how did it move its arms and legs? how did we not see the spike every time it threw its head? how did it SNEEZE? how did the professor know where he was going?)