Suppose you have some Foo class that consists of invalid JS. When you try to instantiate a Foo instance, JSC will complain that Foo doesn't exist, because it was never defined due to the syntax erorr. If you have Pause on Uncaught Exceptions enabled, what happens is that it will pause on the constructor callsite without showing the parse error in the sidebar. Only once you continue will the actual error show up in the sidebar. I'm not sure if it's stuck in a nested runloop or what the cause is, but it makes debugging a parse error hard.
<rdar://problem/20800102>