A couple years ago my dentist sold her practice to a new dentist.
Before I had an appointment with the new dentist, my wife had an appointment, and he told her she had gum disease with bone loss and needed a deep cleaning and to go on a "periodontal schedule" where she would have to come in every three months instead of six. (Note: My wife and I have different last names and separate insurance, so he wouldn't have known we're related.)
When I went in, the hygenist measured my pocket depths, and I noticed she seemed to be adding 1-3mm to what I remembered the measurements being every time I had that done in the past. The dentist then came in and gave me what sounded like a rehearsed speech concluding in the same diagnosis he had given my wife.
I told him I'd like to think about it and could he please send me the records they based this on. They proceeded to rush me out of the office, outright refusing to do the regular cleaning.
I went to another dentist and only told her I left my previous dentist because he gave me bad vibes. She started off with the pocket measurements which were magically back to normal! She said my teeth looked great. Then she asked about my previous dentist and I told her what happened. She said: "I suspected it was something like that."
So... yeah... what an industry...
ETA: Although now in my paranoia I wonder if the new dentist somehow knew the previous dentist's diagnosis and realized the way to keep me as a customer would be to tell me they were wrong. But they shouldn't be able to get my past records unless I authorize it, right?