I had the great privilege of knowing the late Captain E.C. Spencer, chief pilot for Zantop Air Transport until his retirement.
He was a B-24 instructor and USAAC test pilot who worked at Willow Run during the war. Each and every plane that came off the line was inspected, tested, and flown by Army Air Corps pilots and aircrew.
Yes, mistakes were found, but they were corrected before the planes were accepted by the Air Corps, and then turned over to civilian and military ferry pilots to deliver the planes to their new units.
Most errors were small, like missing or badly set rivets, but sometimes they were more serious, like rudder or aileron control cables installed backwards.
The pilots who had to fly these planes for the first time were understandably VERY careful.
CptA