It is kind of like when you have a power loss while writing to a file system (like say FAT32 or EXT2) and when it comes back up you have to do a fsck or equival

It is kind of like when you have a power loss while writing to a file system (like say FAT32 or EXT2) and when it comes back up you have to do a fsck or equivalent. If something gets messed up it may not come back in an consistent manner. Something similar happens at and SSD level. One of the most complex aspects of an SSD to implement and test.