If a mechanic leaves a rag in the works of your vehicle's motor, chances are you'd find out about it before you got it out on the road where something dangerous could happen. Unfortunately, the same can't always be said if a surgeon leaves something behind in your body during an operation.
Doctors might not like being compared to auto mechanics, and perhaps they have a point. If a surgeon makes the medical mistake of leaving something inside your body, the repercussions are potentially more troublesome. The very least that may happen is that you'll suffer an infection that requires more treatment. The worst case scenario is that you might die due to complications.