An instance is reporting errors like this in a clustered repository:
Command failed with error #128! COMMAND git grep --extended-regexp --null -n --no-color -e '' 'X' -- 'Y' STDOUT (empty) STDERR fatal: bad object X at [<phutil>/src/future/exec/ExecFuture.php:369]
...where X is the hash of a valid commit which should be reachable (and Y is a path, although this probably isn't particularly relevant).
This is consistent with synchronization before a read simply failing.