I have a ListBox in my WPF app that is based on a grouped CollectionView binding. It has worked well for a year or two now. I installed VS 11 Beta which installed .Net Framework 4.5. I began seeing unusual behavior with the SelectedItem in the ListBox where multipliple items would becojke selected even though the SelectionMode is Single. This occurs most often after manually setting the SelectedIndex or SelectedItem. However, this is not the only case that causes the bug. NOTE: I DO NOT HAVE DUPLICATE ITEMS IN THIS LIST. Setting the SelectedIndex to -1 before setting the SelectedIndex to a valid value did not help. This does not happen on machines with standard .Net Framework 4 installs, only on those with .Net Framework 4.5. After a miserable day long, error ridden process of uninstalling VS 11 & .Net 4.5 and SQL 2012 and repairing VS 2010 / .Net Framework and VS 2010 SP1, I can confirm that the app now behaves as normal. This is also confirmed on another test machine with .Net 4.5. Current I have 0 machines with 4.5 so I am not in a good position to test further.