Due to bad RAM (now fixed), my (single disk) root partition got corrupted and 
no longer mounts with "corrupt leaf, bad key order":

    root@archiso ~ # mount /dev/sdh2 /mnt
    mount: wrong fs type, bad option, bad superblock on /dev/sdh2,
        missing codepage or helper program, or other error

        In some cases useful info is found in syslog - try
        dmesg | tail or so.

    root@archiso ~ # dmesg | grep -i btrfs                  
    [    8.513028] Btrfs loaded, crc32c=crc32c-generic
    [    8.513033] BTRFS: selftest: sectorsize: 4096  nodesize: 4096
    [    8.513035] BTRFS: selftest: sectorsize: 4096  nodesize: 8192
    [    8.513037] BTRFS: selftest: sectorsize: 4096  nodesize: 16384
    [    8.513039] BTRFS: selftest: sectorsize: 4096  nodesize: 32768
    [    8.513041] BTRFS: selftest: sectorsize: 4096  nodesize: 65536
    [    8.514927] BTRFS: device label btrfs devid 1 transid 36706 /dev/sdh2
    [    8.515047] BTRFS: device label data01 devid 1 transid 645 /dev/sdg1
    [  546.218691] BTRFS info (device sdh2): disk space caching is enabled
    [  546.218697] BTRFS info (device sdh2): has skinny extents
    [  546.225228] BTRFS critical (device sdh2): corrupt leaf, bad key order: 
block=75182981120,root=1, slot=0
    [  546.225303] BTRFS error (device sdh2): failed to recover balance: -5
    [  546.253399] BTRFS: open_ctree failed

Trying to repair the FS gets into a loop of two alternating bad blocks but 
never succeeds:

    root@archiso ~ # btrfs check --repair /dev/sdh2
    enabling repair mode
    Checking filesystem on /dev/sdh2
    UUID: e961bc76-09bb-4616-9263-dab61b8d0ef6
    checking extents
    bad key ordering 0 1
    bad block 75182981120
    Errors found in extent allocation tree or chunk allocation
    bad key ordering 0 1

    root@archiso ~ # btrfs check --repair /dev/sdh2
    enabling repair mode
    Checking filesystem on /dev/sdh2
    UUID: e961bc76-09bb-4616-9263-dab61b8d0ef6
    checking extents
    bad key ordering 0 1
    bad block 75183144960
    Errors found in extent allocation tree or chunk allocation
    bad key ordering 0 1

    root@archiso ~ # btrfs check --repair /dev/sdh2
    enabling repair mode
    Checking filesystem on /dev/sdh2
    UUID: e961bc76-09bb-4616-9263-dab61b8d0ef6
    checking extents
    bad key ordering 0 1
    bad block 75182981120
    Errors found in extent allocation tree or chunk allocation
    bad key ordering 0 1

    root@archiso ~ # btrfs check --repair /dev/sdh2
    enabling repair mode
    Checking filesystem on /dev/sdh2
    UUID: e961bc76-09bb-4616-9263-dab61b8d0ef6
    checking extents
    bad key ordering 0 1
    bad block 75183144960
    Errors found in extent allocation tree or chunk allocation
    bad key ordering 0 1

    [etc...]

According to smartctl, the disk itself (a quite new Samsung SSD) is healthy.

This is not a critical machine, but I would prefer to solve this issue over a 
new install. (I'm quite new to btrfs, though)

TIA
   Heinz