and now I present to you- Beauvais Cathedral; The would be mother of all cathedrals.
Started in the 13th century, it is actually unfinished after twice, structural problems caused collapses. It is the highest-vaulted cathedral in Europe, and WOULD have been the tallest cathedral in the world, had a too-ambitious tower not collapsed.
part of it was never finished. it has structural issues remedied by metal beams and such, (notice those steel cables holding the flying buttresses all around)
the lightest part of the planse, the nave, were never finished. ( I hope they do someday. I mean hell, the Sagrada Familia basilica in Barcelona, Spain started construction in 1882, and is estimated to not be completed until 2026)