Gitaxian Probe

Pay two life and the cantrip costs nothing: no mana, no card, no net resources. That arithmetic is the whole design problem. Looking at a hand is almost incidental flavor wrapped around what the card really does, which is replace itself the instant it resolves while leaving your hand size untouched. For most decks the information is a footnote; for the ones that built around it, the relevant text was "draw a card, costs nothing." It became a fixture in graveyard and storm strategies that count spells cast, fuel delve, or care about cards in the bin, where a free spell that thins toward your business pieces and feeds the graveyard on the way is worth far more than the two life. Those interactions, plus the way it smooths goldfish-perfect combo decks without costing a slot or a mana, are why it has been banned across multiple constructed formats. The design lesson is durable: a spell that costs nothing and replaces itself is not balanced by its visible effect; it is balanced (or not) by whatever the deck around it is allowed to do with a free trigger. The printed effect looks harmless because it almost is. The danger lives entirely in the count of spells cast, and there the card pays nothing to add one.