Looks like there might have been some disturbance/damage earlier in the leaf's development. Keep an eye on the other circular spots, they are common and probably nothing but there is a fungus that sometimes spreads (in which case they would get bigger rather quickly). All in all the plant looks healthy to me. Looks like it may need a repot come Spring.

Looks good to me as well. Small blemishes do happen.

Thank you, woodnative and Stush! Well, that's good news and bad news, isn't it?

You may be right about the leafs being physically damaged. In fact, I had new blinds installed last summer, and had to use them often and hastily (as the facade of our house is getting a makeover, which means strange men right in front of our third-story windows and lots of noise, which my dog does not find amusing at all). Btw., that also means that my plants got much less light... :(

But these circular spots worry me. I'm always paranoid about pests or diseases spreading to other plants, as I don't have another room (with natural light) for quarantine. So I tend to panic about these things. When I was new to cacti, I once threw away a beautiful Matucana Madisoniorum because of what I later learned were very common cork spots...

Now I wonder, if the other rosette is really free of these spots, should I cut it off, repot and discard the spotty one?

If the spots bother you a lot, you could separate the sport but that's not to say the same thing won't happen to it. So many of my Sans have random spots, I don't even notice them anymore. Lol.

I wouldn't discard the spotty one either. You could either use the leaves to propogate it (though the striatuon probably will not propogate) and make multiples or I'm sure many people would love to have a dooneri, spots or not. Whatever you choose, wait for spring to do it.

That dooneri looks beautiful Lisbeth! Can't beat a happy Sans about to flower! Please post pics of the flowers as well when it opens.