@Override
  public void postCompletedDeleteTableAction(ObserverContext<MasterCoprocessorEnvironment> ctx,
    TableName tableName) throws IOException {
    if (!deleteFailedOnce && !tableName.isSystemTable()) {
      deleteFailedOnce = true;
      throw new IOException("execute postCompletedDeleteTableAction failed once.");
    }
  }
A bad Master Observer to prevent user to create/delete table once.
hbase-server/src/test/java/org/apache/hadoop/hbase/coprocessor/BadMasterObserverForCreateDeleteTable.java