This article lists known issues in the RadMultiColumnComboBox control.
RadMultiColumnComboBox does not support design-time at the moment.
When you open the Design-time surface of Visual Studio, RadMultiColumnComboBox instances will not render and will throw the
Failed to create designer error.
RadMultiColumnComboBox supports only the Default skin.
Setting a different skin name will result in a runtime error like
Telerik.Web.UI.RadMultiColumnComboBox with ID='RadMultiColumnComboBox1' was unable to find an embedded skin with the name 'WebBlue'. Please, make sure that the skin name is spelled correctly and that you have added a reference to the Telerik.Web.UI.Skins.dll assembly in your project. If you want to use a custom skin, set EnableEmbeddedSkins=false..