Inspection- save time and make money!

It is rare, but I have been in several situations where I have made some, if not a lot, of progress on a bicycle job, only to find that the work will be fruitless due to a fatal issue with the bike. After working on hundreds of bicycles, it's easy to become complacent with the assumption that most bikes are structurally sound. As with the often ignored rim wear issue, it is easy to miss or over-look a cracked or damaged frame.

A few embarrassing examples:

1. A customer dropped off his son's entry-level mountain bike for a tune up. Try as I might, I could not get the linear-pull brakes to center on the rim. The rim was true and had been "dished" correctly. The pads were spaced correctly. The spring tension was balanced. Finally, it struck me for the first time- the frame! I placed our trusty Frame Alignment Gauge (a surprisingly cheap tool), on the frame and discovered that it was twisted. Oddly, the customer was incredulous that it could have been damaged, "My son just got home from his freshmen year at college where he was just riding that bike to class..."
2. I performed a fairly detailed tune on an older aluminum road bike, only to discover that they NDS chainstay was cracked cleanly in half right at the dropout weld. The customer got some sort of warranty reimbursement, which he hopefully put toward a new bike from my store, but I wasted about two hours of labor on the bike because I did not check the frame for cracks first.
3. My store ordered a whole new drivetrain and other components for a customer's beloved old touring bike. After the magical box appeared, I got to work, eager to see this bike reborn. Sadly, I found a small crack in the chainstay behind the bottom bracket. The frame was toast and the shop had to send all the part back to the distro at our expense.

cracked frame is cracked

This is all to say: inspect all bikes thoroughly before making grand claims about how awesome the bike is going to be when you are done with it. To be clear: it is in the shop's best interest to refuse to do any work on a bike with a cracked or compromised frame. Use your judgement, and don't put yourself in a situation of liability when a bike shatters under a customer, causing injury or death, because your expert opinion neglected a compromised frame.