One of the most common, Brainstorm, allows you to draw three cards and then place any two from your hand on top of your deck in the order of your choice, all at instant speed for one mana
In other words, you can put 5 counters on The Ur-Dragon, but you don't win the game that way
Any creature the player controls that couldnt be targeted by the original spell (due to shroud, protection abilities, targeting restrictions, or any other reason) is just ignored by Mirrorwing Dragons ability
As soon as your deck is at all redundant Treasure Cruise is simply superior
Hell, some decks will happily make a 5-0 split, knowing that 5 cards in the yard and a 5/5 flier is just as serviceable as 5 cards in hand
Black Lotus can be tapped and sacrificed for three mana of any one color and is one of Magics few 0-cost cards that are exceptionally powerful