Historical Rules · 17 July 2015
Rule 601.2e
Official source text from the 17 July 2015 Rules release
Based on the previous announcements, the game checks to see if the proposed spell can legally be cast based on applicable timing rules (including ones based on the card’s type) and other effects that may allow a spell to be cast or prohibit a spell from being cast. If the proposed spell is illegal, the game returns to the moment before the casting of that spell was proposed (see rule 717, “Handling Illegal Actions”).