Follow this steps to reproduce:
- set a really small indexing queue capacity for Solr by setting solr.indexer.queue.capacity in xwiki.properties
- delete the Solr index (e.g. data/solr folder)
- start XWiki, access a page and wait a bit then perform some searches
You can easily notice that the search results are incomplete (and it's not because the documents have not been indexed yet).