What's the worst thing they can find in a colonoscopy?

So as not to alarm you first I will say colonoscopies are done to look for a variety of things. Such as gastrointestinal problems and the like. So don't ume the worst. At the same time the procedure is very invasive so its possible another simple procedure didn't clearly determine what problem he had if any. Also you didn't offer your fathers age. But at a certain age a colonsocopy will be suggested just as a preventive measure. But to answer your question a colonscopy will find something like colon cancer since usually while they are in there scoping they take biopsies. Hopefully it is being done just as a preventative measure in your dads case. Good luck to the both of you.