Human error.
When Boeing were planning the 777, they were going to offer a version with folding wingtips so that it could fit onto narrow gates. Some people said it was a bad idea because one day 777 would try and get airborne with the wingtip folded. An american pilot wrote in to Flight magazine and agreed. He said that once people got used to seeing airliners taxying around with wingtips folded, no one would think it unsual and it would probably lead to one trying to get airrborne in such a state. He was writng from a position of authority as he had actually got airborne from a carrier in an A7 with the wings folded - and managed to land safely back on the deck!