The problem is that when you have SearchMode=Contains , and have item that contains the entered text and one that starts with this text, the ComboBox selects the one which contains the text(as it is with smaller index) but scrolls to the one that starts with it.

When using similar code as below, RadComboBox does not select the specified item. If the DomainDataSource data was already loaded, the selection is successful.
<telerikInput:RadComboBox ItemsSource="{Binding ElementName=MineTypeDomainDataSource, Path=Data}"
  SelectedItem="{Binding Path=MineType, Mode=TwoWay, NotifyOnValidationError=true, ValidatesOnExceptions=true}"> 
 <telerikInput:RadComboBox.ItemTemplate> 
  <DataTemplate> 
   <TextBlock Text="{Binding MineTypeName}" HorizontalAlignment="Left" /> 
  </DataTemplate> 
 </telerikInput:RadComboBox.ItemTemplate>  
</telerikInput:RadComboBox>  
<riaControls:DomainDataSource AutoLoad="True"
  x:Name="MineTypeDomainDataSource"
  QueryName="GetMineTypesQuery"
  LoadedData="MineTypeDomainDataSource_LoadedData"
  DomainContext="{StaticResource CitationDBSvc}" />

Create a RadComboBox, bound to a collection of objects, implementing INotifyPropertyChanged. Use DisplayMemberPath. Select an item, then change the display member property of the selected item - the selection box is not updated.
As workaround you can use DataTemplate:
<telerikInput:RadComboBox HorizontalAlignment="Left" Margin="122,52,0,0" VerticalAlignment="Top" MinWidth="150" x:Name="radComboBox" telerik:TextSearch.TextPath="Name">
    <telerikInput:RadComboBox.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding Name}" />
        </DataTemplate>
    </telerikInput:RadComboBox.ItemTemplate>
</telerikInput:RadComboBox>

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

Available in LIB Version 2016.3.1010, it will be also available in the 2016 R3 SP1.

The issue can be reproduced also with the standard ComboBox:
https://connect.microsoft.com/VisualStudio/feedback/details/663870/wpf-4-0-combobox-itemtemplate-and-selectionboxitemtemplate-when-itemtemplate-is-bound#details

Workaround: use OnDropDownClosed event.

Available in the 2016 R3 Release.

Please check our help article for more information: 
http://docs.telerik.com/devtools/wpf/controls/radcombobox/features/multiple-selection

When the CheckBox is focused the issue is appearing.

Available in LIB version 2016.2.905, it will be also available in the 2016 R3 Release.

If there are plenty items in RadComboBox and you double click an item to select it sometimes the wrong item is selected because of the animation.

The same issue can be observe when placing the control inside ToolBar.

When you click on the TextBox of an editable RadComboBox all the text should be selected, but it is not, even when SelectAllTextEvent="All" is set. The text is blinking as selected for half a second and then deselects.

When filtering is enabled, enter text that doesn't exist in the items and when the focus is removed the text entered is deleted, but it should stay.
The solution of this issue was implemented by a new feature in the control which determines if the Text of the RadComboBox should be updated on LostFocus or not.
Simply set the UpdateTextOnLostFocus dependency property of the control to "False". By default its value is True and no change in behavior should be observed.
The UpdateTextOnLostFocus dependency property is available in the 2015 Q2 SP1 release.

Place a RadComboBox control in a RadPane. Make it floating and dock it again. When selection is changed, the SelectionChanged event is fired multiple times.

RadComboBox and RadRibbonComboBox in their NonEditable mode do not reflect background set inline. A style for the ToggleButton contained in the NonEditable templates prevents the TemplateBinding from working properly as it uses a hard-coded background. For RadRibbonComboBox it is a Metro theme-only issue.

IsEditable=True and ReadOnly=True

Properties like AutomationProperties.Name are not read if the RadComboBox is editable.

RadComboBox with IsEditable = true, IsReadOnly = true and IsFilteringEnabled = true. Select an item and then in order for the filtering to work you have to delete the selected text with Backspace