Endless loop?

I've seen three cases now where adding a simple calculation on user input causes some kind of endless loop. After the user enters data in that field, the little "working" animation starts turning and never stops. Here's the latest example of a line that, when added, causes this behavior:

input.Price_Total_Net = (input.Price_Total_Retail - input.Price_Total_Retail * input.Supplier_Commission / 100);

Thanks,
Randy