Add SelectedItemsSource property which can be bound to a collection in the view model. In this case when the RadListBox is in Multiple/Extender mode, the selection can be manipulate from the ViewModel.

Implement IsSynchronizedWithCurrentItem property in order to disconnect the connection with CollectionViewSource.

When the control is initially Collapsed setting the SelectedIndex does not update the SelectedItem property of the control and vice versa.

The SelectionMode must be Multiple.

If a dragged item is not over an existing item of the ListBox control the drop cue is not displayed. By default it should be rendered after the last item in the ListBox control.

Available in the 2014 Q2 Release.

The SelectionMode must be Multiple.

Available in LIB version 2015.3.1019, it will be also available in the 2015 Q3 SP1. Fix provided by the new IsScrollIntoViewEnabled property.

When the ItemsPanel is set to VirtualizingStackPanel which Orientation is set to Horizontal, selecting an item moves it to the rightmost position.

Available in LIB version 2016.3.1017, it will be also available in the 2016 R3 SP1 release. 

When the ItemsPanel of the RadListBox is RadWrapPanel and the user press Down/Up key the RadListBox selects always the next/previous item instead of the corresponding item on the next/previous row.

As a workaround, the ScrollingSettingsBehavior.ScrollStep attached property of the RadListBox control can be set to a value equal to that of the Height of the RadListBoxItems.

Available in R3 2017 Official Release Version.

Use case that hits the described issue:

When binding another control to a shared SelectedItem property from a ViewModel causes the Item to not be updated correctly to the ListBox and the anther control.

Using an attached property to populate the SelectedItems of the control initially selects only the first item.

A sample project (ListBoxSelectedItems - Example) of the used approach can be found in this blog post:
http://blogs.telerik.com/xamlteam/posts/13-01-14/extending-the-functionality-of-radcontrols-with-attached-properties

Available in LIB version 2016.3.1017, it will be also available in the 2016 R3 SP1 release. 

Available in LIB version 2016.1.321, it will be also available in the 2016 Q2 Release.