Do I need to pay for repairs that didn't fix the problem?

What is your question?

I recently took my Durango into the mechanic because even though the engine would turn on, as soon as I took my foot off the gas, it would stutter and die. Prior to this happening there had been no issues. It had been driving normally, was parked in the drive way for a few hours then wouldn't run correctly. The mechanic initially told me it was the spark plugs and wiring and replaced them accordingly. Afterward, he said the "fix" actually didn't fix the problem and would continue to investigate further. A few days later he called to tell me he had found a broken piston and that I needed to replace the whole motor. How did we go from worn out spark plugs to needing a whole new motor? And should I pay for the repairs that didn't fix the problem?