Careening (referred to in animations as "BulletMode", referred to by SSBWU/3DS tips as cannonball effect) is a state entered by defeated enemies in the Subspace Emissary, Smash Run, and Master Fortress. Once its HP reaches zero, a blast sound can be heard and the defeated enemy begins flying in a straight direction (based on the angle of the attack that hit it), bouncing off parts of the stage and damaging other enemies it runs into. Upon reaching a blast line or after a few seconds the enemy is removed from play.
The damage dealt by a careening enemy is dependent on the difficulty, with higher difficulties making the attack weaker. Generally, larger enemies deal more damage when careening - a careening Greap would deal massive damage on any difficulty, while a careening Mite would barely deal any damage, even on easy mode. Careening enemies also always use a special knockback angle encoded as 363, whose properties are mostly unknown.