I've got a form containing (among other things) 3 dropdowns, each with about 5-10 choices. The choices in these dropdowns are closely related and some of the permutations will be invalid. This also means that if the user changes the selection in one of the dropdowns then the current values in the other two may be invalid.
My design doc currently states that "if an invalid combination is chosen then an error should be displayed" and "to help the user to pick valid combinations all choices in the dropdown lists which will produce an invalid combination should be low-lighted in the dropdown".
I'm not convinved that this is the best way of dealing with this but I can't think of any better alternatives. I thought about combining them into a single dropdown containing all of the valid combinations but this could be unwieldy as well.
Does anyone have any other solutions?