They sell you something you don't need. This is the worst kind of rip-off. This occurs regularly in certain shops.
Protect yourself: Have them show you each needed item before and after repair; get written estimate; get second opinion before; verify repairs after; get old parts.
They don't know what they are doing. Many unskilled techs are in the field due to a general shortage of technicians nationwide. You are paying for their incompetence: wasted diagnosis time, mistakes, and parts not needed.
Protect yourself: Select a shop, which is accredited by AAA, ASE and ASA. Also make sure your tech is ASE certified in the specialty he's working in. Just because a tech is certified in one specialty, doesn't mean in all areas. Test their recommendations with a second opinion.
They promote a price which isn't realistic. Cheap 4 wheel brake jobs: $99, but really averages $200-$300. Scheduled maintenance, tune-ups, and many other services can also be misleading.
If it sounds too good to be true, it probably is!
Have an honest shop give you a correct estimate or get ready to open your wallet much wider than you thought!