@Override
  protected long append(RegionInfo info, WALKeyImpl key, WALEdit edits, boolean inMemstore)
    throws IOException {
    if (this.ft == FailureType.APPEND) {
      throw new IOException("append");
    }
    return super.append(info, key, edits, inMemstore);
  }
This is a utility class, used by tests, which fails operation specified by FailureType enum
hbase-server/src/test/java/org/apache/hadoop/hbase/wal/FaultyFSLog.java