On the Legacy / Silverlight Designer, using the right-click shortcut to Copy / Paste results in an Error.
Note this is only an issue if you use the right-click shortcut to paste inbetween two existing activities.
You cannot paste by using the right-click shortcut between two activities. This causes a "corrupt" canvas and a reload of the solution is needed to fix it.
This is a known issue and only affects Legacy / Silverlight designer.