Upon Maria's return to the VonTrapp's home while the children are singing, "My Favorite Things", Maria puts down her suitcase to hug the children. Fredrick (one of the children) goes behind Maria and picks up the suitcase. There is a break in the film (another angle) and you see Fredrick picking up the suitcase a second time. That is the slip-up.
I'm not completely familiar with this film or anything, but there are a couple of possibilities. First off, they could break to something happenening at the exact moment he went to pick up the suitcase, and upon returning to that scene, he's still there showing what he was doiing. It is also equally possibly to consider that while the camera moved away, the boy was distracted (as many people are) by whatever the camera was focused on then, and after recomposing himself, went to pick the suitcase back up again.
In this scene, the children run up to Maria. Kurt (younger of the boys) is carrying a jacket. Maria rubs him on the head and he leans down to pick up her bag. Friedrich goes around behind her and picks up the guitar. There is a slipup like you are referring to, but it's Friedrich and the guitar. It is not Kurt and the suitcase.
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