Something I've been seeing unusually often lately: use of "-im" words as singular.
The "-im" suffix, in words derived from Hebrew, is generally a masculine plural, as far as I can tell (I'm sure Shmuel or others will correct me if that's wrong). So words like "cherubim," "seraphim," "Nephilim," "dybbukim," "Hasidim," "kibbutzim," "klezmorim," and "goyim" are plural.
In English, other plural forms are often acceptable. For example, it's fine in English to say "cherubs," "seraphs," "dybbuks," and even "goys." (We usually talk about the Nephilim in plural; I don't think I've seen "Nephil" singular.)
But in all those cases, it's not correct to use the "-im" forms as singular. * "A Nephilim walks into a bar" is grammatically wrong; likewise * "Wow, that cherubim is totally hot."
I imagine that part of the confusion comes from Madeleine L'Engle's A Wind in the Door, in which there's a character who's referred to as a cherubim. But even there, L'Engle was aware that that's nonstandard:
Calvin made a sound which, if he had been less astonished, would have been a laugh. "But cherubim is plural."
The fire-spouting beast returned, "I am practically plural. The little boy thought I was a drive of dragons, didn't he? [...]"
A Wind in the Door, p. 56 of (I guess) the 1974 Dell edition
Of course, this is only an issue for -im words that come from Hebrew. For example, "victim," "verbatim," "grim," "disclaim," "denim," and "Sondheim" are not plurals.