On December 5, 2018 the Falcon 9 booster started spinning during the landing burn, and promptly appears to have soft landed on the ocean. While SpaceX stated that the booster could be reused it never was. But the question remains, what went wrong? Did it have to do with the small thing that flaked off? Did the grid fins fail?
The Falcon 9 booster landing failure during the CRS-16 mission was caused by a grid fin hydraulic pump stall. The grid fins are used to steer the Falcon 9 booster during its descent back to Earth for landing.
During the CRS-16 mission, one of the hydraulic pumps that control the grid fins stalled, causing the fin to be stuck in a certain position. This led to the booster spinning out of control and ultimately crashing into the ocean instead of landing on the drone ship as intended.
SpaceX later announced that they had upgraded the hydraulic pump systems on their Falcon 9 boosters to prevent similar incidents from happening in the future.
SpaceX stated that the booster could be reused it never was.
I don’t sure where SpaceX said it could be reused. Maybe just after they recovered because according to Wikipedia:
It failed to reach Landing Zone 1, but recovered enough to achieve a water landing off Cape Canaveral. Shortly after the landing, Elon Musk, CEO of SpaceX, stated the booster appeared undamaged and was being recovered. After recovering the booster, it was found to be too damaged to fly again and was scrapped for parts.