Wikisimpsons - The Simpsons Wiki
An unnamed Amish Man visited the Simpsons while the family were building their swimming pool.
When the Simpsons bought a swimming pool from Pool Sharks, assembly was required, so the whole family (even Maggie) pitched in to put it together. When they finished, they stepped back to admire their handiwork, and it was revealed (with the family not yet having realized it) that the family had built an aluminum barn rather than a swimming pool. Homer said, "All right. Everybody in the pool!"
At that point, the Amish man appeared, stood next to Homer, and said, "'Tis a fine barn, but sure 'tis no pool, English." When it dawned on Homer that they had indeed built a barn, not a swimming pool, he exclaimed "D'oh-eth!" rather than his usual "D'oh!" After that, the Simpsons had another go at building the pool and succeeded on the second attempt.
 Behind the Laughter