I don't spend much time on the JVM, but in ClojureScript, this is what I get for a similar test (including multiple definitions of foo in my case, i've elided the whole source file here):

     (defn foo [a b] :hi)
     (defn bar [] (foo 1 2 3))

     WARNING: Wrong number of args (3) passed to prepost.core/foo at line 25 src/cljs/prepost/core.cljs
     WARNING: foo at line 24 is being replaced at line 48 src/cljs/prepost/core.cljs

It's a compile-time warning even when the app is not running, but ClojureScript goes through a different compilation step than JVM Clojure. Perhaps I was mistaken in the JVM case, or someone else can chime in.