The current behavior is to "trim" the ID or name from the final path element before sending the key to the server:
https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-datastore/src/main/java/com/google/cloud/datastore/DatastoreImpl.java#L114

This means you can call allocateId on a key with a prepopulated ID, and the server will actuall allocate a completely different ID. This behavior is documented, but it seems surprising. Would be safer to either reject the allocateId call if the provided key contains an ID/name (or just let the server reject the call).

Reference:
http://stackoverflow.com/questions/41878706/where-is-allocateids-in-the-google-cloud-datastore-java-library/41879429