Inode numbers are stored in big endian format on disk, and host endian format in-core.

Applications that pass 64 bit inode numbers using 32 bit variables will truncate the 32 most-significant bits.

Since XFS stores the AG number an inode belongs to in the most significant bits, a result of this truncation can be an inode number that points to an inode in a lower AG by mistake.

Using that inode number will result in either a lookup on the incorrect inode, or the referencing of an area on disk that doesn't contain inodes at all.