As a former mechanic, at least my shop would occasionally refuse to let a vehicle leave the shop- the one example that I remember most clearly was a SUV that came in for an oil change. The owner was planning on a 800 mile trip with their family and wanted an oil change before leaving. One of the wheels had a serious suspension problem, and would likely fall off shortly & cause a rollover.

We weren't going to let that truck leave our shop w/o getting fixed, none of us wanted dead kids on our conscience. We extended credit to the customer & cut our rate so the owner could afford it. Point being, in this case we definitely knew better than the customer, somehow they didn't realize a wheel falling off at highway speed is dangerous. P

However, that was a very rare situation. If a customer insisted they wanted work done that we didn't feel was necessary, we would tell them so but still do the work if they wanted it.