I see stuff like: "merge problem with lucene 3 and 4 indices" (from solr users list), and cannot even think how to respond to these users because so many things can go wrong with IndexWriter.addIndexes(Directory)
it currently has in its javadocs:
NOTE: the index in each Directory must not be changed (opened by a writer) while this method is running. This method does not acquire a write lock in each input Directory, so it is up to the caller to enforce this.
This method should be acquiring locks: its copying RAW FILES. Otherwise we should remove it. If someone doesnt like that, or is mad because its 10ns slower, they can use NoLockFactory.