ALTER TABLE .. CONCATENATE should fail if the table is not managed by hive.
For unmanaged tables, file names can be anything. Hive has some assumptions about file names which can result in data loss for unmanaged tables.
Example of this is a table/partition having 2 different files files (part-m-00000_1417075294718 and part-m-00018_1417075294718). Although both are completely different files, hive thinks these are files generated by separate instances of same task (because of failure or speculative execution). Hive will end up removing this file
DDL should restrict concatenation for unmanaged tables.