New Twiddle

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
	init() {
    this._super(...arguments);
    throw 'a little exception';
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  count: 0,
  actions: {
    increment() {
      this.incrementProperty('count');
    }
  }
});

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.error {
  color: red;
}

templates.application.hbs

<p>
	A reproduction of <a href="https://github.com/glimmerjs/glimmer-vm/issues/484#issuecomment-359464769">this issue</a>.
</p>
<h3>the count is {{count}}.</h3>

<button onclick={{action 'increment'}}>+</button>

<p>
	This component will cause an exception in the vm. Clicking the button above will then result in a <span class='error'>Uncaught Error: a glimmer transaction was begun, but one already exists. You may have a nested transaction</span> error.
</p>

{{my-component}}

templates.components.my-component.hbs

your name is: {{data.name}}...

twiddle.json

{
  "version": "0.13.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.13.0",
    "ember-template-compiler": "2.13.0",
    "ember-testing": "2.13.0"
  },
  "addons": {
    "ember-data": "2.13.2"
  }
}