DF-3049 console evidence (baseline stock module, three mount attempts):

run A (fresh boot, mount attempt):
WARNING: mount of vn0 denied due bad gd=57 csum=0x4e5, expected=0x7c2e - run fsck

run B (after vn detach/attach cycle -- zeroed pages behind gd alloc):
WARNING: mount of vn0 denied due bad gd=63 csum=0xdead, expected=0x58ac - run fsck

run C (after newfs+mount+umount churn on /dev/vn1):
WARNING: mount of vn0 denied due bad gd=57 csum=0x4e5, expected=0x3c11 - run fsck

offline (in-bounds-only, zero-padded) computation for group 57: 0x04e5
 == the stored value the kernel accepted for groups 0..56 and compared at 57.
kernel 'expected' values differ per mount and from 0x04e5 => the crc input
 included bytes beyond the 4096-byte e2fs_gd allocation (live kernel heap).