That sounds to me like (1) not a clear example of a Gettier case and (2) something that would, in fact, be rare.

#1 because one of the reasons for your believing X has cancer is that "he has the symptoms", which (if I'm understanding your example correctly) is in fact a consequence of the cancer he actually has; so, at least to some extent, your belief is causally connected to the true thing you believe in just the way it's not meant to be in a Gettier case.

#2 because (so far as I know) it's not at all common to have both (a) cancer in an organ that doesn't show up in your X-ray (or MRI or whatever) and (b) a cyst in the same organ that looks just like cancer in the X-ray/MRI/whatever. I'm not a doctor, but my guess is that this is very unusual indeed.

So this isn't a very convincing example of how clear-cut Gettier cases aren't rare: it's neither a clear-cut Gettier case nor something that happens at all often.

In the general case it's "I believe X based on signs that would imply X correctly, and I happen to be correct that X holds, but I misread the signs I used to come to the conclusion".

I don't think this is rare -- including in the version of my example.

The only reason there's arguing that it's not a "clear cut case" is that I mentioned seeing "symptoms". Ignore the symptoms I mentioned, as they are a red herring, e.g. seeing the mark could cause the belief alone.

Other than that, it's a belief (1), that's justified (2), and true (3) -- while being accidentally justified.

Consider the case of a policeman that things someone is dangerous because they think they seen a gun on them. So they shoot first, and lo and behold, the suspect did have a gun on them -- but what the policeman seen was just a cellphone or something bulky under their jacket.

Or the spouse that thinks their spouse is having an affair because they see a hickey. Their spouse indeed has an affair (and even has a hickey on the other side of the neck), but what their spouse saw was just some small bruise caused by something else.

Or, to stick with the theme, figuring domestic abuse, and the victim suffers that indeed, but your guess is based on a bruise they had from an actual fall.

As in, when u write "I can very easily think of examples in medicine, police work (e.g. regarding suspects), accounting, and so on..." but you don't give any, the natural tendency is not to believe you.

That would be the "natural tendency" if I was describing something mysterious and rare that few can fathom. Whereas to me those situations don't seem really as far-fetched or hard for someone to come up on their own.

I didn't write "one can easily" to imply I have some special talent to imagine such situations (and thus had motive to leave examples off to hide the fact that I don't).

I wrote it because I really do believe one can easily find such examples, and wasn't it even worthy to go into details (since I mentioned medicine, police work, etc, I thought the cases I implied where pretty clear too).

In any case, I gave 3 examples in a comment above.