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A recent question (on $\ce{CI}$ v $\ce{Cl}$) reminded of this question: are there any real (as opposed to contrived) formulae which would be the same if case was ignored?

One certain example is $\ce{Co}$, Cobalt, and $\ce{CO}$, Carbon Monoxide. Of course, confusing Cobalt and Carbon Monoxide is rather unlikely.

In my search for others, the best example I could find was $\ce{BaS}$, Barium Sulphide, and $\ce{BAs}$, Boron Arsenide. Are these real substances and is the stoichiometry just $1:1$? I had trouble verifying that.

Are there any more interesting examples?

One real example that I saw once was a poster in a university chemistry lab showing some complex organic molecules. $\ce{Ac}$ appeared at many points but it seemed very unlikely that these compounds contained Actinium. I then noticed $\ce{Me}$ and $\ce{Et}$ and guessed that they were organic units e.g. methyl and ethyl so $\ce{Ac}$ may have been acetyl.

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    $\begingroup$ Well, this is a legitimate concern indeed, which is why we feel irate to no end when somebody messes up cases in chemical formulae. $\endgroup$ – Ivan Neretin Nov 26 '18 at 13:29
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    $\begingroup$ BaS is indeed a known compound, with the NaCl prototype. BAs is also a known compound with the ZnS prototype. $\endgroup$ – Jon Custer Nov 26 '18 at 19:53
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    $\begingroup$ Well I never put it together that actinium acetyl have the same abbreviation. $\endgroup$ – A.K. Nov 27 '18 at 3:53
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Letter case in chemical names sometimes matters too! E.g. when o-methylphenol (o-cresol, ortho-cresol, 2-methylphenol, I) improperly capitalized to O-methylphenol (instead of o-Methylphenol), it can be interpreted as a different compound, phenol methylated at oxygen (anisole, methoxybenzene, II).

Fig.1

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