Natural language is the use of the language as the people using that language normally perceive its meaning. "Diagraming" the sentence to show how its meaning varies "technically" from natural language is pointless. Natural language use often not proper and people say and write things "incorrectly" all the time but the meaning is naturally understood.
In the two examples I wrote, people using English in normal context would not see a difference between those two statements, as (at least)
@LadyElect seems to understand that (but then went on to change the wording with a different example, which isn't how either feature is written...). To say "that is" in this context simple means "in the process of."
Anyway, so
@Parmandur and
@dave2008, can Vecna use Dread Counterspell on someone NOT casting a spell? Because according to "your diagraming" he should be able to, all that matters is he "sees a creature". Frankly, IMO that's ridiculous since the purpose of the feature is to stop spells and the natural language makes "interrupt a creature he can see
casting a spell" what is important really. After all, if the creature isn't casting a spell, Vecna makes no check, and the creature takes no damage.
Other than those two phrases, "that is" and "in the process of", the features core are the same: "interrupt a creature (he/you) can see casting a spell". The only real difference is Vecna can use it without the 60' limit.
I have a minor in English and have taught English at University level while in the Peace Corps in Europe (Ukraine, specifically) and a master's in Education and a Master's in Mathematics. I
know language (ESPECIALLY ENGLISH) can have meanings "twisted" just like statistics can. When you get down to technically analyzing statements, you are going beyond the natural use of the language.
Not a "mistake" so much as just a different use of figures of speech and wording. There are many ways to convey an idea using slightly different wording, but the meaning is the same.
Now, let me ask this: if Dread Counterspell was written "who is" instead of "that is", would that make any difference?
Did I?

It doesn't really sound like something I would do, especially since I don't really care that much, so maybe it was someone else? Or it might have been me... You'd have to dig up the post--I don't recall it anyway.