Continuity mistake: When Bart, Lisa, Homer and Marge are on the ferris wheel near the end, we can see in one shot the person behind Lisa has blonde hair, and then in the next shot the person has black.
Audio problem: When Bart gets the beavers to knock down the tree so that Clara can walk across, she gets onto it and starts walking gently. But when she walks gently, it sounds like she's walking quickly across the trunk.
Continuity mistake: Before Bart notices the lemon behind the lemon-shaped rock there is actually no lemon there. We can later see that there is too much of a significant gap between these two objects for the lemon to be hiding behind the rock.