When we rename the file with overwrite flag as true, it will delete the destination file blocks. After deleting the blocks, whenever it releases the fsNameSystem lock, NN can give the invalidation work to corresponding DNs to delete the blocks.
Parallaly it will sync the rename related edits to editlog file. At this step before NN sync the edits if NN crashes, NN can stuck into safemode on restart. This is because block already deleted from the DN as part of invalidations. But dst file still exist as rename edits not persisted in log file and no DN will report that blocks now.
This is similar to