It's really awkward, but I'm going to point it out anyway:
Muriel Bowser, the mayor of the District of Columbia, pronounces the name of her territory as "Dishtrict of Columbia."
I'da thunk someone mighta tole her by now to get that problem fixed!! If, that is, if it is, indeed, a problem...

Harry B.

And then there's also the OAS - the Organization of American States. Many many writers use "OAS's" as the possessive of that organization's initialism-with-an-s-ending-and-plural-third-word.

Two really good examples - cuz they come from *huge* government bureaucracies, which wouldn't *allow* bad punctuation - are the CMS, Centers for Medicare and Medicaid Services (mentioned above, I think), and its "parent" organization, the HHS, Health and Human Services. Both of these, in their press releases, use the 's construction: "CMS's" and "HHS's". But maybe that counts as only *one* example, because I suspect the same people may be the copy editors at those two places, or they learned everything they know from each other.

IRS's, BCS's, CBS's, and probably dozens more. But I think I may have relaxed the definition of what you were looking for, Willie from Warsaw.