The use of "alumni" in the singular. A person is an alumnus or an alumna, the alumni are always a group. Seems to be a very American usage, and I don't know why it feels aggravating where other Americanisms like positive anymore don't.
Conversely I hate the trend in English language to keep the original pluralization rules when adopting words from other languages. Just anglicize it to "alumnuses" or "alumnis" (if you want that to be the singular). Rules of the original language don't matter any more when you use it in English.