I was able to find an explanation (and create a patch) for the compilation issue in reagent (but it probably affects other projects) when using parallel-build: http://dev.clojure.org/jira/browse/CLJS-1614

It should be preventing many projects from using paralllel-build... @darwin it would be great if you could try compiling that project of yours that was breaking with parallel-build using this patch (I can push a branch with the commit to my cljs fork if that makes it easier to try it)