If you are trying to use ASP.NET AJAX functionality in combination with Master pages you will get a JS Error 'get_postBackElement().id' is null or not an object.'.
Current workaround is quite simple. Just set ClientIDMode property of the Page to AutoID. (ClientIDMode="AutoID").
You can find more on new asp.net features here.
8069d4d6-8d8c-4139-9c33-6d7215ab9c6b|2|5.0