Unknown Xcode Instruments crash
I've seen that Exception before when I throw a Javascript primitive and it reproduces reliably and consistently with my Instruments-4.5 (4523).

Your comments state that this does not reproduce reliably.  Is it possible that your tests do not take the same path reliably (and hence, not consistently throwing), and this is why they only reproduce sometimes?  Before continuing, I suggest verifying that your tests are taking the same path consistently.

To verify throwing a Javascript primitive is the case with your tests, do these steps.

Find a Javascript test file that reproduces with your InstrumentsException.

Verify the Javascript test throws a Javascript primitive, as opposed to a Javascript Object.  Specifically, you want a line that uses "throw" but does not use "new" in the same line.  The relevant line should look like this,

throw "Who's your daddy";   //Instruments throws an InstrumentsException with this.


not 

throw new Error("Who's your daddy?");  //Instruments has no problems with this.


Verify that your installation does actually throw that specific InstrumentsException when you throw a primitive.

throw "Who's your daddy";


At this point, you've got a pretty good suspect.  But you to make more certain, try removing the suspicious "throw" line or replacing it with something else, just to see if you can get the test to pass.