"Colonoscopy" is a procedure and "massive tumour" is a diagnosis; they're not mutually exclusive. If someone had an abnormal growth in the rectum or another part of the large bowel they'd be experiencing certain symptoms such as rectal bleeding. Those symptoms would trigger referral for a colonoscopy, but the diagnosis likely wouldn't be known until after the procedure.Back to the codes thing.... I have been through this with private health care in the UK. Your doctor writes down that you need procedure GYN567 and you call the insurer and they say it's a hysterectomy or whatever. In order for the "massive tumour" to be confused with "colonoscopy" the codes would have to be similar - a 8 mistaken for a 0 or a L for a I or similar. If the two codes were totally different then how could that even happen? Unless of course a ABC123 code meant one thing in 1997 and something totally different in 2020.
Medicare codes are basically procedure codes. They represent the service the doctor provides to the patient. There is also such a thing as diagnostic codes, and these are used by hospitals and their funders. A Medicare code (colonoscopy would have been something like 32090) is not going to be mistaken for an ICD-10 code.