GWT and Ext GWT (GXT) are interesting platforms to develop complex web applications using Java. It supports fancy UIs and all client-side code gets compiled into JavaScript.

There are a couple of non-obvious gotchas though that will make your application crash and burn without a clear error message.

1. Make sure you add your GWT/GXT LayoutContainer to a <div>. If you add it to a <p> it will result in an "Unknown runtime error" in Internet Explorer.
2. Make sure that the id of the <div> you are adding your LayoutContainer to is not the same as the name of your GWT module or you'll run into GWT issue 4003.