This post contains spoilers for those who, for whatever reason, haven't yet seen "Inside Out." And now you've been warned.
Pixar's "Inside Out" is a delightful imagining of how our brains process emotion. In the movie, it works like this: Marble-like memories tinted by the emotion(s) that dictated your feelings at the time are delivered to the brain's control center. The ones that glow extra bright are your most important, "core" memories, and they're placed in a special chamber. Your more pedestrian memories get sent down to storage. Some will hang around until they're dumped into a black abyss, where they're destroyed. Others will hang around until they're recalled -- that is, sent back up to the control center to be played in the mind's eye -- by maintenance workers.
But, wait. Joy and Sadness, after they got accidentally stuck far outside the control center with Riley's core memories, ran into those maintenance workers. They watched them locate a memory of that horrible gum commercial jingle, laugh to themselves, and send it up the recall tube to be suddenly and inexplicably played in Riley's head. Joy made the connection, and tried unsuccessfully to hitch a ride inside the tube herself, lugging the bag of memories.
Why couldn't Joy have taken the core memories out of the bag and sent them up by themselves to the control center, where at least they'd be safe?
Was she such a terrible micromanager that she couldn't trust any of the other emotions to handle it? Sadness, who'd freaked everybody out by leaving her gloomy mark on the core memories she'd touched, was locked out of the control center with Joy. That is, she wasn't a threat. Was Joy afraid that, if any of the other emotions got their grubby little paws on Riley's memories, they'd alter them in a similar way?
"Inside Out" director Pete Docter is, indeed, aware of the issue. When asked about the plot hole, he quipped, "Yeah, well then we wouldn't have a third act," before explaining how the idea of recalling memories was added in later, "box[ing] [the screenwriters] in a corner a little bit."
"Our argument was that Joy wouldn't trust the memories would be fine on her own; she needed to be up there too," Docter told Rope of Silicon earlier this month.