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If the target creature is an illegal target by the time Code of Constraint tries to resolve, the spell doesn't resolve. You don't draw a card.
Code of Constraint can target a creature that's already tapped. If you cast it during your main phase, that creature won't untap during its controller's next untap step.
Addendum abilities of instant spells apply while the spell is resolving, not immediately after casting it. If the spell is countered, you don't get the addendum bonus.
If an effect copies a spell with an addendum ability while it's on the stack, the copy wasn't cast at all, so you won't get the addendum bonus.
