ERROR: left link changed unexpectedly

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Hi all,
in my development server running a 7.4.5 I can replicate
continuously this error:

ERROR: left link changed unexpectedly

I obtain this with a vacuum full:

test=# vacuum full verbose ua_user_data_exp;
INFO: vacuuming "public.ua_user_data_exp"
INFO: "ua_user_data_exp": found 361471 removable, 20867 nonremovable row versions in 28373 pages
DETAIL: 0 dead row versions cannot be removed yet.
Nonremovable row versions range from 432 to 1060 bytes long.
There were 0 unused item pointers.
Total free space (including removable row versions) is 218782956 bytes.
26761 pages are or will become empty, including 0 at the end of the table.
27543 pages containing 218591140 free bytes are potential move destinations.
CPU 1.73s/0.40u sec elapsed 17.66 sec.
INFO: index "exp_id_provider" now contains 20867 row versions in 1077 pages
DETAIL: 0 index row versions were removed.
1003 index pages have been deleted, 1003 are currently reusable.
CPU 0.05s/0.06u sec elapsed 4.87 sec.
INFO: index "exp_ci_login" now contains 20867 row versions in 1832 pages
DETAIL: 0 index row versions were removed.
866 index pages have been deleted, 866 are currently reusable.
CPU 0.18s/0.02u sec elapsed 12.96 sec.
^[INFO: index "exp_country" now contains 20867 row versions in 1341 pages
DETAIL: 0 index row versions were removed.
1187 index pages have been deleted, 1187 are currently reusable.
CPU 0.11s/0.05u sec elapsed 5.89 sec.
INFO: index "exp_os_type" now contains 20867 row versions in 1333 pages
DETAIL: 0 index row versions were removed.
1237 index pages have been deleted, 1237 are currently reusable.
CPU 0.05s/0.03u sec elapsed 5.84 sec.
INFO: index "exp_card" now contains 20867 row versions in 1417 pages
DETAIL: 0 index row versions were removed.
1288 index pages have been deleted, 1288 are currently reusable.
CPU 0.09s/0.02u sec elapsed 6.14 sec.
ERROR: left link changed unexpectedly

last lines in the logs ( with verbose on ):

Oct 1 21:35:31 porto postgres[31356]: [1268-5] LOCATION: vacuum_index, vacuum.c:2728
Oct 1 21:35:38 porto postgres[31356]: [1269-1] ERROR: XX000: left link changed unexpectedly
Oct 1 21:35:38 porto postgres[31356]: [1269-2] LOCATION: _bt_pagedel, nbtpage.c:888

I don't care to fix the problem immediatelly, so if you want dig on it let me
know.

Regards
Gaetano Mendola

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