Lack of accountability for damaging your car

I took my car in on the 12th of November for service, it was fine all along just due for a service. Upon return, my car was jumping and vibrating upon accelaration after changing gears. I thought it was the battery or something, so I took it for diagnostics and realized that it was a misfiring from plug 4. I called them to report the issue and they said they didn't do any plug changes. I told them my car was fine before it went to them and they said I should contact MFC since they are paying my service. What's sad is that if it was a problem before the service, they would have flagged it on the report sheet as they did with the other parts. Not to mention it didn't have a problem before I took it then. I'm so disappointed and will never take my car there ever again.