In release notes to latest release you can find:
Fix VACUUM's tests to see whether it can update relfrozenxid (Andres Freund) In some cases VACUUM (either manual or autovacuum) could incorrectly advance a table's relfrozenxid value, allowing tuples to escape freezing, causing those rows to become invisible once 2^31 transactions have elapsed. The probability of data loss is fairly low since multiple incorrect advancements would need to happen before actual loss occurs, but it's not zero. In 9.2.0 and later, the probability of loss is higher, and it's also possible to get "could not access status of transaction" errors as a consequence of this bug. Users upgrading from releases 9.0.4 or 8.4.8 or earlier are not affected, but all later versions contain the bug. The issue can be ameliorated by, after upgrading, vacuuming all tables in all databases while having vacuum_freeze_table_age set to zero. This will fix any latent corruption but will not be able to fix all pre-existing data errors. However, an installation can be presumed safe after performing this vacuuming if it has executed fewer than 2^31 update transactions in its lifetime (check this with SELECT txid_current() < 2^31).
What does it really mean?