I agree that you should always do a quick pre-ride check, so it is partially your fault but it also never should have left the shop that way. That being said, people are people and because of that they make mistakes on occasion. When I was working in the shop we made every effort to make sure each bike that left was something we would feel perfetly safe on but we did have one or two slip through with something wrong over the years. Nothing bad every happened thankfully, but each time we felt absolutely terrible. So, while it's unfortunate, it's not necessarily the sign of a bad shop in my oppinion. Even the very best of mechanics can have a bad day, get distracted, etc. and let something slip once in a while.