I don't have a minimal repro, but this commit https://github.com/clojure/clojurescript/commit/86f26bf267f1c729313320c38e53e64450c0e68d somehow causes a regression on a dual-hexacore box when compiling with :parallel-build true with the compiler throwing https://gist.github.com/mfikes/408fb49010a3bce8bc35b151411ee738 FWIW, the stack traces are extremely reminiscent of a similar issue that occurred with Transit previously (https://github.com/cognitect/transit-java/issues/20), but that was resolved and I'm using transit-clj "0.8.297" downstream, which has the fix. Will try to track it down, or failing that, I'll start buying all ClojureScript contributors dual-hexacore boxes.