loose cannon

Short definition from Wiktionary

Noun

1. 1
   A cannon that breaks loose from its moorings on a ship during battle or storm, with the potential to cause serious damage to the ship and her crew.
2. 2
   An uncontrolled or unpredictable person, especially considered liable to cause unintentional or indiscriminate damage.

Verb

1. 1
   To behave in a way such as to cause unintentional or indiscriminate damage.