I had to modify the source of the CMSListMenu so it would generate the proper code for a flyout menu in IE (using conditional statements). However, when I copied the dll's over to the bin directory I started receiving an error like the one in the subject.

'CMS.CMSHelper.CMSContext' does not contain a definition for 'ViewMode'

I fixed a few of them by putting

CMSContext.CurrentUser.ViewMode

but is there a reason why this is happening?