How about "He is a loose cannon" ?

Comes from the old sailing battleships on which the mouth loaded cannons needed to be pulled back to relaod and than push forward out of their ports to fire. The cannons were on wheels therefor and kept in check by many ropes (ooops lines) and pully blocks. Sometime a cannon brok loose and the rolling of the ship in the waves would run the heavy cannon on wheels all around over the gundeck or just straight through the wood work on the otherside taking everything with it. Try to stop on of those ! Hence "loose cannon" = Big trouble


Wouter