If something happens accidentally, it didn't happen on purpose — blame it on a mistake, chance, or outside forces.
There are things you mean to do, like brushing your hair or buttoning your shirt. Then there are things you do accidentally: you didn't mean for them to happen, but they happened anyway. Just about all spilled milk happens accidentally. If a surgeon leaves a scalpel in a patient, let's hope he did it accidentally. And if a football bounces off a defender into the running back's arms and he scores a touchdown, he made a great play — accidentally.