It also an American dialectal pronunciation. I grew up, in Chicago, saying both “PEE-kahn” and “pih-KAHN” so interchangeably that I forget which is the “normal” pronunciation in America. Plus I sometimes would say “PEE-can” and there’s also “pee-KAHN,” according to that map.
Here we venture into the unsavory territory of Doctor-Speak, where any and all mispronunciations are accorded equal validity. How many SONT-i-meters is their doo-oh-DEE-num from their um-bi-LYE-cus ?
Don’t get me started !
I was greatly embarrassed by this one as well. I can’t even guess how many years I read this name and internally pronounced it as HER-mee-own, including while reading the Harry Potter books. At some point I made reference to this character in conversation, and decades later my brother still won’t let me forget it.
Funnily enough, I was just now doing a ‘Nine Letter Word’ puzzle, and the nine-letter word was BESEECHED. And I’m damned sure it’s a word I’ve never heard spoken at any time.
Another is ennui.
Are you sure it was Pan-nosh? IME it would normally be P’n-arsh , to rhyme with harsh.
I went to a restaurant called Automaton (Aww-TOM-a-tonn) some years ago and asked them why they called it that; I don’t think they actually answered my question, they were so pleased I said the word properly - not “auto-may-tonn”.
on-WEE. You must not hang out with enough existentialists or morose characters. 
Do you have non-rhotic accent? Because “panache” definitely does not rhyme with “harsh” in most US accents, as there’s no “r” in it. Explaining it to an American as “p’n-arsh” will yield unexpected results.
Maybe “haash” would be a better rhyme - “pan-aash”.
Well, dang, I’ve been using most of these words for 2 decades, and some of them since the early 1980’s, and I realize I have no idea how to pronounce any of them, except, possibly “sysop” which was explained in an early Commodore magazine.
Geoduck is one that surprised me when I finally heard it said out loud.
Also: Schuylkill. Depending on where you are from, it can vary. SKOOL-kill or SKOOkill.
I have never, ever heard it pronounced that way in my decades of IT work.
When I lived in Philadelphia, the expressway was pronounced SURE-kill.
Do you also call top lip facial hair a mous-tock?
My cousin’s in local county government IT and he pronounces it “sigh-sop”. When I was BBSing in the late 80s/early 90s, that seems to be how a lot of people pronounced it, before settling on “siss-op.” I’m also one of those people who say “jif” for GIF, though.
I’ve heard many of these words (duodenum, detritus) pronounced differently by people who are “experts” on the topic.
I spent years thinking the clothing brand Jordache was pronounced “jor-dake,” when it’s actually “jor-dash.”
How about “plethora”, as in “A plethora of people have completely missing the point of the OP”.
Several words from Lovecraft come to mind, such as “Oleaginous,” “Eldritch,” and “Cthonian.”
Antediluvian, Atavistic, Bachtrachian, Cacodaemoniacal, Effulgence, Gibbous, Gloaming, Ichthyic, Lucubration, Obdurate, Piliferous, Proboscidian, Rugose…
Did he perhaps speak more in Seurat than in Ingres?
If you’re a stickler, in the original Greek it’s octo(=eight)+podes(=feet), so I’ve always understood it as octo-PO-des. But “octopuses” follows regular English so that’s OK, but never “octopi” which is neither Greek nor Latin.