Why does my calculator have a dark smudge on it that looks internal and won’t to away. Sorry for lack of better description.
You may have bent the screen inward too much and changed the shape of it, causing some of the pixels to act funny. If it’s not much of a nuisance, I’d say just ignore it. But if it covers parts of the screen, its time to either send it back to the manufacturer for repairs, or just get a new calculator.
Yeah it’s not very large. Is it common or does it mean I didn’t take good care of my calculator? 
(Sorry for late reply) I can’t say for sure, I havent owned that many calculators in my life, but I can guess its pretty common, people can accidentally crush their calculators in their backpacks or something along those lines.