mocha is not your friend if you want to test uncaughtException handlers

excerpt.js

      before(function (done) {
        /**
         * Mocha is extremely zealous about trapping errors, and runs each test
         * in a try / catch block. To get the exception to propagate out to the
         * domain's uncaughtException handler, we need to put the test in an
         * asynchronous context and break out of the mocha jail.
         */
        process.nextTick(function () {
          // disable mocha's error handler
          mochaHandler = process.listeners('uncaughtException').pop();

          agent = helper.loadMockedAgent();
          var disruptor = agent.tracer.transactionProxy(function () {
            domain = agent.getTransaction().trace.domain;
            active = process.domain;

            active.once('error', function (e) {
              json = agent.errors.errors[0];

              return done();
            });

            // trigger the domain
            throw new Error('sample error');
          });

          disruptor();
        });
      });

      after(function () {
        // ...but be sure to re-enable mocha's error handler
        process.on('uncaughtException', mochaHandler);
      });