EDIT: Now that I think about it there is another one that is hard for me, even it I know it is the correct form in English: visa (singular) and visas (plural). In my mother tongue it follows the same pattern as datum and data, it's Visum (singular) and Visa (plural).
Wha? Oh, you mean quae navis navigaverunt.
This applies to the singulars that denote single objects too: scissor lift, pant leg; not scissors lift or pants leg.
According to this principle, it should be datum point, where only the head noun point may be pluralized.
"Datum point" exists, but I've never seen or heard it in use; it's just a "confirm by Google".
Were you also commenting on its pronunciation? I'd instead say: "data" is a mass noun like "mahyonnaise" and "tomahto sauce".
Which contexts did you have in mind?
See also: Meccano, Duplo, Brio, Scalextric.
Is "gimme the lego over there" strange? I think it is, and if it is, then it's an interesting dater point to support this hypothesis.
"Forums" (rather than fora) seems a little awkward to me, but "data point" seems more natural than "datum". But then I never learnt Latin at school. At some point we need to stop applying the grammar rules of foreign languages to fully adopted words.
There is also the noun vīsum, -ī, plural visa with the primary meaning the seen [thing], appearance, perception. This is the word that was borrowed into English and other languages.
And 'in agreeance' instead of 'in agreement.'
Think coffee - you have one coffee (singular), two coffees (plural), and some coffee (an indeterminate amount).
One datum, Two data, Some data.
"people" is the plural form of "person", i.e. one person, two people, as a group
"people" (pl. "peoples") is a group of people as a unit, e.g. a tribe or an ethnic group or the citizens of a country
"persons" is ALSO the plural of "person" but only used when you talk about individuals rather than a group (e.g. "person of interest" -> "persons of interest", not "people of interest")
Yes this is a bit awkward and yes sometimes what's grammatically possible still sounds awkward even to native speakers (e.g. I'd argue "the polices" is a legitimate term when discussing multiple police organisations of a country as an abstract group but it sounds extremely wrong to most people apparently).
For example, there is a UN Convention on the Rights of Persons with Disabilities. Obviously, native speakers would say "people with disabilities" in almost every situation, but that would be too informal for the title of a treaty.
Likewise, law enforcement use formal speech such as "persons of interest" to bolster their authority, but nobody would ever say "I sat next to two Australian persons on the flight".
Admittedly, it's more likely I'd say "I sat next to two Ozzies on the flight." (No, not Aussies, Ozzies. Like Ozzy Ozbourne. Because that's how it sounds to me when Aussies say 'Aussies.')
I appreciate the apparent strangeness of this, but it's certainly not unique within the language.
That's English! :)