From the Super Mario Wiki
An Invincibility Barrel (also called an Exclamation Point Barrel and an Invincible Barrel) is a rare type of Barrel that has appeared in the Donkey Kong Country series, first appearing in Donkey Kong Country 2: Diddy's Kong Quest. These wooden barrels feature an exclamation mark, and instantly break when they are touched. Upon being broken, the barrel grants the Kongs temporary invincibility for a few seconds and the Kongs can defeat enemies just by touching them, similar to the effects of a Super Star from the Super Mario series. Invincibility Barrels appear infrequently throughout the games that feature them.
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