This article is from the Magic: The Gathering Rules FAQ, by Patrik Linell (pls@claymore.nu) with numerous contributions by others.
It's what happens if you add mana to your pool but don't use it
before the end of the phase. The mana disappears, and you lose 1 life for
each point of mana that disappears in this way. It doesn't happen very
often, but sometimes you really need to play something costing BB with only
one Swamp and a Dark Ritual, meaning you have one black mana left over.
 
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