Well first of all ClassicConsistencyHybridUpdateSemanticsSolrCloudTest (in the patch) is a test that protects the fix from being broken, because it depends on the issues mentioned in the description above being fixed. So if you commit the entire patch attached to SOLR-3178 you will have the fix for the issues AND a good test (even though its core focus in not the issues in this SOLR-3428) protecting if from being broken again by accident in the future.
One reason that you probably never found the issues before now, is that it is really hard to detect that requests are forwarded (from non-leader to leader or from leader to replica) multiple times or not at all, if requests always result in idempotent-that-cant-fail-operations only is carried out on server-side. And that has always been the case for update requests until I introduced versioning and unique key constraint errors. Using those features update requests are no longer idempotent and fail-free - an update that succeeds nicely the first time it is carried out will not succeed if it is carried out a second time (DocumentAlreadyExists-, DocumentDoesNotExist- or VersionConflict-error will occur). Because of the fact that updates are idempotent and fail-free, and because we have not finegrained (per document) and typed error propagation (before my patch), it will be kinda hard to construct a (not too artificial, with a lot of mocking in order to simulate unrealistic behaviour) test showing those issues mentioned in the description above (and protecting the fix from being broken in the future). So I guess I would rather not try.
Instead I really encourage you to have that patch committed. It provides well done features and fixes. I know it is a big patch covering many issues and I have learned for the future to try not to bring so big patches, but basically all the issues are kinda related - versioning and the ability to fail on unique key constraint violations (SOLR-3173 and SOLR-3178) is really not worth much without the ability to report the typed errors back to the client per document (SOLR-3382) and the bugs in this SOLR-3428 is kinda harmless and therefore hard to provoke in a test as long as everything behind an update request is idempotent and fail-free (which it is before SOLR-3173, SOLR-3178 and SOLR-3382). I really think it is worth it that one or two of you committers spent a day or two understanding the features and fixes in the patch - it will save you much more than the one or two days later, if you have to make the features and fix the bugs yourself.
So bacially I think it would be a waste of (a lot) of time trying to create a poor (because it will have to be very artificial and unrealistic) test for the issues in this SOLR-3428. IMHO you should put your effort into doing whatever it takes to get the big patch committed.
Regards, Per Steffensen