Failure cost

From CMC Wiki

Failure Cost refers to an additional, usually negative, effect when an entity's ability or a cast spell fails. This cost does not take place if the spell or ability cannot be cast (such as through a lack of mana, or the casting monster is destroyed).

In many cases, the failure cost is part of the effect (for example, if Life Eater has its ability fail, you will still take damage); this is indicated by the placement of a colon in the card text. The portion of the ability with appears before the colon is a cost which will be incurred regaurdless of success or failure. Additional failure costs are usually denoted after the ability text with a Failure: tag infront of it. (Ex: Cardboard Cutout has the text Failure: Dizzies, which means it dizzies if something causes its ability to fail)

Most counterspells have an unique failure cost called which reverses the effect of the previous counterspell in the same resolve phase (called an 'Uncounter'). This basically causes two counterspells to neutralize each other's effect. See the Counterspell article for further detail.