Patrick Hunt noticed this. Without the global block cache, the block cache was not reused after a directory was closed. Now that it is reused when using the global cache, leaving the underlying entries presents a problem if that directory is created again because blocks from the previous directory may be read. This could happen when you remove a solrcore and recreate it with the same data directory (or a collection with the same name). I could only reproduce it easily using index merges (core admin) with the sequence: merge index, delete collection, create collection, merge index. Reads on the final merged index can look corrupt or queries may just return no results.