If anything goes wrong, the owner will sue you, the GC, the city, and the guy who takes out the trash. If you make a document stating someone else is liable for anything you do, it will be an office joke on its way to the trash can. You need errors and omission insurance, period.
Through no fault of my own, I have been brought in on a few lawsuits. Typically the owner sues anything that moves. A team of forensic engineers will scour your plans and look for any tiny mistake, they will do the same for whatever got installed in the building. They will put a micrometer on pipe walls, threads, and grooves, check tolerances of anything fabricated, demand calibration certificates from your fab shop equipment, measure every stick of pipe and compare dimensions to your drawings. They will validate your drawings and calculations and if anything is off, you will answer for it.
None of these lawsuits have ended up anywhere and nothing I've designed was found faulty. However it is expensive to get sued and jump through these hoops. Even if you're right you still need insurance.