From DF Wiki:
Minecarts are not without danger. Although designating a track automatically sets the traffic designation to low, dwarves may still walk on them, and creatures ignore traffic designations altogether. If an unlucky dwarf or creature fails to dodge a minecart, they can be injured. Most of this danger can be avoided by setting the minecart hauling commands to guide instead of push or ride, as dwarves guiding minecarts will ignore traffic restrictions, by pasturing domestic animals, and preventing the access of other creatures to the tracks.
Danger does not always involve living victims: careless route designation can also result in minecarts careening off tracks or colliding with each other. If this occurs, the items may be scattered; this can cause even more hauling jobs than the minecart aimed to eliminate. Even
better worse, scattered items, especially weapons, can injure passing dwarves or other creatures; in the words of Toady One the Great, "Accidental grapeshotting of the dining room should be possible now."
Of course, the danger of using minecarts means they can also be used as weapons by imaginative players.