There is a word, which is I'm 99% sure of ancient Greek ancestry, which means something like 'pedantic' except that the person it is being used to describe is actually unknowingly ignorant about what he is being arrogantly pedantic about.

Example: I used the phrase 'leit motiv' in a letter-to-the-editor. He printed my letter but changed 'leit motiv' to 'leit motif'. 'motif' is in fact wrong, being French, whereas 'leit motiv' is clearly German, and 'motiv' is the correct spelling here.
I wrote the guy and told him a bit contemptuously that he had made a xxxxxxx, this being my lost word. Can soembody help me restore this useful word to my vocabulary?

Thank you

'leit motiv' is clearly German, and 'motiv' is the correct spelling here

Actually, in German, the word is Leitmotiv. The word in English is sometimes spelled leitmotif.

In re OP: Pedascule or grammaticaster?

he had made a...., it is a noun you're looking for, not an adjective as your introduction suggests?

solecism, faux pas, gaffe... (see Bartleby)

None of them quite conveys point I think you're trying to emphasize, which is that by making the correction the perpetrator is actually betraying his ignorance. My favorite example is the lady from Sinclair Lewis' Main Street who tried to display her erudition by pronouncing "hoi polloi" as "wa pollwa" -- she thought it was French!

open mouth, insert foot . That a local idiomatic expression? Strange.....

The definition of "to put your foot in your mouth" here includes a element of embarrassment at having said the wrong thing unintentionally, perhaps trying to be helpful but actually making matters even more awkward. Admittedly that's not quite the same as the original situation here, where the "helper" was displaying ignorance and not even caring.

Thank you wofa, when you imagine this visualised it is quite an impossible act. (Good to have that Bartleby service in the sideline , grazie)

“Judged strictly by the standard of his own time, Bacon’s ignorance of the progress which science had up to that time made is only to be equalled by his insolence toward men in comparison with whom he was the merest sciolist”.
- Thomas Huxley, Fortnightly Review (1878)

but sciolist/sciolism stems from Latin. link

aside: this reminds me of one of my favorite sesquipedalianisms:
macroverbumsciolist
1) a person who is ignorant of large words
2) a person who pretends to know a word, then secretly refers to a dictionary
robotman

also, Shakespeare coined (or found) pedascule, a contemptuous diminutive of 'pedant' (see TotS).

Googling sciolist and pedascule yields, for me, one ghit. It is from a very strange website in Academe (link). (OTOH, googling grammaticaster and pedascule gives results of a different kind.)

the one ghit (1st case) only exists because sciolist(s) and pedascule are both nine-letter words. this is the sort of site that Anu (and googlewhackers?) would find fascinating. <g>

the 2nd case is entirely too self-referential autogenous for my tastes. :p

I think my head just asploded.

so don't go all Strong Bad on us, eta.
-joe cool

How do you even know about that?!? Watch out for the Burninator..... TROGDOR!!!!

Yes, yes, yes - 'sciolism' is indeed my lost word! Many thanks indeed.
I first met the word at least 15 years ago in a "Unusual Words" feature published once-a-week in the London Times.
It is a dandy word to have in one's lexical arsenal - in this day and age of widespread pontification by semi-, quasi- or pseudo- literates!

I hope this gets to you, Tsuwm - thanks again