In "Billy and Mandy's Big Boogey Adventure," Numbuh 3 promised to get back at Grim, but when Grim is captured, they don't even remember each other or the events of the TV movie.
Note: It's possible events from this crossover occur before the Big Boogey Adventure even though the latter aired first.
( Reply: When Numbuh 3 appeared she said, "Hi, guys! It's me, Numbuh 3!" This proves that The GAOTKND happened first. )