That is why I do all of my maintenance. I've heard of mechanics pulling a brokenn part off the shelf and showing it to the customer and saying, "this is your problem!" Charging them and doing nothing.
My dad owned a car dealership, I paid his mechanic to re-do my hubs on my SUV and it still had a problem so I pulled it apart myself to see and it was full of old grease with some new stuff slopped in there. Boy was his job on the line!
I have a good mechanic on hand, but nothing is worse when ladies get all sorts of BS form mechanics and I go in with them to "understand" what they are talking about. And the story changes and the price goes down a couple hundred dollars.