1 Kyros: Well, I have 200 mana ready now. I have to do something with it or it'll dissipate...
2 Lambert: That doesn't sound so bad.
2 Kyros: ... explosively.
2 Lambert: Oh.
3 Mordekai: You could cast a different spell.
4 Kyros: Fireball is the only one I know at this high a level!
4 Mordekai: Of course...

It's more usual for unused mana to dissipate harmlessly in roleplaying games (the card game Magic: the Gathering treats it differently...), but Kyros won't be having any of that!

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2016-02-29 Rerun commentary: Actually, since this comic was originally published, the rules of Magic: the Gathering have changed, and in fact in the current rules unused mana now dissipates harmlessly as well!

Originally, the game had a rule called "mana burn", in which unused mana would damage you, causing one point of damage for each point of unused mana. But this was changed when the rules were revised in July 2009, so that unused mana no longer caused damage.

I'm just glad that Mark Rosewater listens to me, even if it does take him three years to incorporate suggested changes from this comic into the Magic rules.

Ghazbán Sceptre - 3
Artifact
At the start of your upkeep, Ghazbán Sceptre does 3 damage to you, then the player with most permanents in play takes control of Ghazbán Sceptre.
1: Destroy target permanent you control.

You saw it here first.