MVCC in PostgreSQL: visible tuple with xmin > current txid
This depends on the isolation level. The default level for postgresql is "Read Committed" at which level this can happen. In postgresql it won't happen at "Repeatable Read" or "Serializable".

This behavior is often not a problem and higher serialization levels can slow down the server which is the reason why they are not the default.

For an extensive explanation please see the manual https://www.postgresql.org/docs/9.6/static/transaction-iso.html