I actually have a test harness - https://github.com/hferentschik/hsearch-faceting-perf - I just never have seen this before. The harness is geared towards comparing Lucene faceting against Search faceting, but still it is executing faceting multi threaded. I've never had an issue. Since I am planning to continue with some faceting related issues, I'll extend the tests and try to see whether I can trigger a concurrency issue.