You have to remember dealer misdiagnosis is not with volvos alone.. All dealers misdiagnois problems.. Yes you would expect when you bring a car in for service that they will know exactly what the problem is a fix it the first time. Mechanics are people and see things differently from their past experiences. I myself opt to do repairs myself because of past experiences.
Any thing that I cant (because of tools or otherwise) I try to find out first and then bring it to the mechanic to repair or replace the faulty part.
If I leave it upto the mechanic to fix first time then its a 80% chance that it will cure the problem that they diagnose.

Thats why brickboard is an EXCELLENT resource for diagnosing problems and issues and i fully research any issues with my vehicle. Its probably save me thousands! On my last car I misdiagnosed (non volvo) and ended up replacing the whole front end. And no mechanic would have found the problem either which would have cost me more since the problem an item that would never need replacement.

Overall the best thing is to be well informed and to narrow down any problem with your car before entering any garage. Spend some time here at the brickboard and research because all the answers are here.