> My doctor requested that I get a diagnostic colonoscopy. Because I needed one under my insurance plan I had to pay the full cost, over $3k. If I didn't need one it would be a "screening" colonoscopy, and covered in full by the insurance.

A similar, but at least nowhere near as expensive, gotcha in many US insurance plans happens with annual checkups. Many plans fully cover an annual check up with no deductible or co-pay.

But during that checkup if you mention some issue that the doctor does not specifically find or ask about as part of the standard checkup, you might find that your insurance company counts that visit as a diagnostic visit or a treatment visit, not a checkup visit, and you pay the normal office visit deductible or co-pay.

If you ask something that takes significant time for the doctor to deal with that might make sense, but I’ve read of it happening for things like you tell the doctor that you get sore wrists after a full day on the keyboard and the doctor spends a minute telling you about taking breaks and giving you some ergonomics tips.