What happens when the {X} in Mind Grind *is* zero?

Question

Normally, when you cast a spell without paying its mana cost, the {X} in its mana cost is 0.

Mind Grind, however, has an additional restriction:

X can't be 0.

Clearly you can't choose for {X} to be zero when you cast it normally. But what happens if you don't get to choose, e.g. when you cascade into Mind Grind or cast it with Epic Experiment?

Answer 1

You must choose zero for X, but you can't choose zero for X, so you can't cast the spell. Epic Experiment doesn't force you to cast anything ("may cast"), so it will be moved to the graveyard by the later part of Epic Experiment's effect.

107.3b If a player is casting a spell that has an {X} in its mana cost, the value of X isn’t defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0. This doesn’t apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, “Casting Spells.”

101.2. When a rule or effect allows or directs something to happen, and another effect states that it can’t happen, the “can’t” effect takes precedence.

601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. Casting a spell follows the steps listed below, in order. If, at any point during the casting of a spell, a player is unable to comply with any of the steps listed below, the casting of the spell is illegal; the game returns to the moment before that spell started to be cast (see rule 717, “Handling Illegal Actions”). Announcements and payments can’t be altered after they’ve been made.

You can see these rules applied by an official source in the Oracle rulings for Ertai's Meddling:

10/4/2004 Ertai's Meddling can't be cast through any way that doesn't pay its mana cost. This is because the X in the Meddling's mana cost can't be 0, but effects that allow spells to be cast without paying their mana costs set X to 0.

Answer 2

If you cascade into Mind Grind, you're simply not allowed to cast it. Cascade is a 'may' effect, and so is Epic Experiment. The situation is essentially identical to revealing something like Flame Slash (4 damage to target creature) with no creatures in play — since the spell can't be played without a legal target (or in Mind Grind's case, can't be played with an X of 0), it's just never cast. In the case of Cascade, this means that Mind Grind will be one of the cards put on the bottom of your library; in the case of Epic Experiment, it means it will be put in your graveyard without having been cast.