There are two issues that generally make some test cases (not just TestOrderPreserveUnionHDFS) in Zebra's "pigtest" fail intermittently.
1) There is some randomness when multiple tables are unioned. The correctness check relies on the ordering of tables in output rows, which is incorrect. Instead the table a particular row belongs to can only be associated with the table index in output;
2) There are some failures in PIG STORE calls as the destination directory are not cleaned up properly before store.