Stable system booting from 5-disk SATA z2 pool. Recently one disk failed/replaced successfully. Eventually system rebooted (for other reasons) and gives error message "ZFS: zfs_alloc()/zfs_free() mismatch" during "rolling wheel" part of boot followed by hard hang (requires reset/power cycle).
I have tried removing the recently replaced disk with no apparent change.
https://www.reddit.com/r/freebsd/comments/p50oun/zfs_zfs_alloczfs_free_mismatch_message_upon_boot/ mentions a different user who resolved the issue by rewriting the bootcode. Is your bootcode up to date? Is that true for any/all disks that have bootcode installed to them?