Final answer:
The sentence lacked parallel structure due to the mismatch in grammatical form between 'learning to fly a plane' and 'swimming.' It was corrected by adding 'to learn' after 'swimming.'
Step-by-step explanation:
The mistake in the sentence "Learning to fly a plane and swimming were two skills Lucy really wanted to learn." involves a lack of parallel structure. The sentence should maintain the same grammatical form to provide clarity and balance. The corrected version could be:
"Learning to fly a plane and learning to swim were two skills Lucy really wanted to learn."
The inclusion of "to learn" after "swimming" aligns it grammatically with "learning to fly a plane," thus fixing the error in parallelism.