gone off half-cocked

abrupt breakneck brief frantic harum-scarum hasty headlong heedless hurried ill-advised impatient impetuous indiscreet jump the gun madcap off the hip off the top of head plunging precipitant quick rapid rash reckless refractory rushing subitaneous swift unanticipated uncontrolled unexpected unforeseen violent willful without warning

careful cautious slow