I hit the following on 9.4 testing logical decoding.
TRAP: FailedAssertion("!(prev_first_lsn < cur_txn->first_lsn)", File:
"reorderbuffer.c", Line: 618)
LOG: server process (PID 3801) was terminated by signal 6: Aborted
Unfortunately I don't have a core file and I haven't been able to
reproduce this.