A colonoscopy perforating the bowel and leading to an infection and sepsis.
These things happen, they are not common but they are not zero-chance events either.
And you have to consider the opportunity costs of consuming the doctor's time, the labwork, and the facilities, possibly delaying treatment for someone else who actually needs it.
Risk of perforation is something like .03% and almost never fatal whereas a colonoscopy reduces colorectal cancer mortality by 60-70%.
>And you have to consider the opportunity costs of consuming the doctor's time, the labwork, and the facilities, possibly delaying treatment for someone else who actually needs it.
That is also rarely true. More often, greater demand for a service soon yields economies of scale, more efficiencies and overall more patients served at a lower price. Low volume is expensive, high volume is cheap.