During target selection, you can cancel spells you shouldn't be able to

If an effect tells you to cast a spell (e.g. suspend), and you splice one or more cards onto that spell as you cast it, xmage lets you cancel while you are selecting targets or paying costs for the spliced card(s). If you do cancel this way, it cancels the entire spell you are casting, even if the effect that told you to cast it was not optional. For example, you can cancel a card coming off suspend this way and the suspended card will just stay in exile forever, with no time counters on it.