GWT/GXT Unable to get value of property 'user.agent'

If your app hangs in IE/IE8 during load and you have

Unable to get value of property 'user.agent'

error under Development Mode, you should instruct your IE how to interpret application's page. To do this, put

<meta http-equiv="X-UA-Compatible" content="IE=8">

in your index.jsp or whatever you use to render the app.

More information on GWT wiki page.