We recently upgraded our main application to the latest version of the Infragistics controls. Everything converted, compiled, and ran fine. However, when you tried to open some forms in the Visual Studio designer, you got the message "The path is not of a legal form". Looking at the project for these forms, they had invalid references to custom controls that used Infragistics controls. I deleted the references and added the newly compiled versions to eliminate the designer rendering error.
Since we do not remove the previous version of Infragistics the older controls are found at run time.