I've seen it, but not in some time. My most recent command usages all include -Sthreads because I added it back when I was fighting with classloaders and the thread the s3 downloaded launches

I'll rephrase that then: I'm a bit surprised I've never seen that at work -- I don't use the -Sthreads option. 🙂

I wonder if it is more likely if your m2 isn't completely populated

Race condition on unsynchronized HashSet?

that is my guess based on similar stacktraces

Oh, I thought I was the only one. I get it from time to time - I just rerun clj and it usually works the next time.