My #1 mentally mispronounced word:
Boadicea.
I have to fight to remember that it’s not pronounced “Bodacia”. As in, “Bodacious” but without the S. “That Boadicea was one bodacious babe!” My mental pronouncer also goes into heavy Bill-And-Ted accent for it, too.
Quite unfitting a Warrior Queen!
Abandoning. Just doesn’t exactly roll off the tongue for me.
First of, I’m not a native English speaker.
“Aisle”, the spelling is all wrong if you ask me.
A-I-S-L-E = ahyle… you gotta be kidding me.
Come to think of it “island” is another.
And don’t get me started on Edinburgh…
I know this isn’t what we’re talking about, but Yeah, I know, riiiiiiiiiiiiight?
I thought Worcestechire was pronounced Werchestuhsheer, and Leicester was Laichester. Also Arkansas. How’s one to know, really? Just experience, I suppose.
Thing I have trouble pronouncing, have to slow down and concentrate- Sutherland. Like the recently deceased Joan Sutherland or the actor Donald/Kiefer Sutherland. My tongue feels laggy and kind of drunk when I say that name.
I dunno why.
Rural.
I really struggle with it. Ruhrrl. Rurrralll. Rrrrrrrl.
My mouth does not want to create the appropriate sequence of sounds for that word.
The phrase “pharmacological teratogens” when I talk about teratogens in my developmental class. I have to slow WAY down to get that phrase right.
The Rural Juror!
Aha, now it’s when being a native Spanish speaker comes handy. I can go RRRRR until the cows come home.
El ferrocarril recorre los rieles rumbo a Rusia. 
Juror is actually easier, if you separate the Rs into separate syllables. Unfortunatly, RUE-ruhl is not an acceptable pronounciation of rural. The best I can do is to never stop the R in the first syllable. Go straight from the initial R[ɹ] to the second R[ɝ]. rrr-ral. Don’t try to make a normal vowel out of the U. There is no or [ʊ].
= boot
[ʊ] = book
ETA: Otherwise, just go with RU-wuhl, dropping the R like people do in February, surprise, and library.
Interestingly enough, the odd spellings of aisle and island are tied together. It all started back in the 15th century when some well-meaning but misguided writers were attempting to make the history of English words more sensible by adding silent letters to words in an attempt to make their derivations from Latin and other languages more obvious. Unfortunately, because of the lack of Latin education nowadays, very few people walk around experiencing moments of epiphany regarding the word debt and its latin originator debitum, but rather they exclaim irritation at that superfluous B hovering around where it’s clearly not wanted. And one of the recipients of this treatment was the word isle, which at the time was pronounced and written without any Ses. However, coming as it did from the Latin word insula, it was decided that the S was needed, at least in writing as a reminder of the origin of the word. Island, despite arising from a different etymological background (a combination of ieg and land meaning, if I am not mistaken, land on the water) received an S as well for the sake of consistency. Aisle, which was hanging out on the outskirts of the ordeal and congratulating itself on avoiding the whole mess, found itself dumped unceremoniously in the middle and getting stuck with a completely unnecessary S merely on the basis of having the temerity to sound like isle. Aisle itself having arisen from the old french ele, meaning wing, as of a church, which came from the latin ala and meant simply wing. The A came even later because of association with the french cognate aile. I apologize if this bored anyone but I found it amusing that the two words Ale chose happened to be connected in such a way.
Don’t. I love this place.
It is not altogether clear to me what you think the correct pronunciation should be, but, in any case, the name Boadicea is now thought to have originated from a copying error in a Latin manuscript. Historians now generally refer to the queen in question as Boudica (pronounced pretty much as it appears: Boo-dick-uh). That may not be exactly what she called herself, but it is certainly a lot closer than Boadicea, however you may pronounce it.
Armageddon. I can actually pronounce it correctly but I heard a guy mispronounce it “armegadon” once and it is absolutely stuck in my head and I have to actively guard against it.
By the way, I didn’t laugh but a little bit of pee did come out.
roar and idea. I grew up with a speech impediment and though it is mostly gone now those two words still give me fits.
That’s simply unapseptigal. unugsactivle. unbespecticalled. unconsfectigalled.
That ain’t right.
I find it works well to think of it like this:
Armageddon getting out of here! (I’m-a gettin’ outta here!)
Lien.
I spent an hour in my friend’s friend’s office - the guy was a lawyer - talking about this lee-en that got put on my house by mistake. I’m sure my friend was mortified that he brought his bumpkin-ass friend (me) to talk to his lawyer friend.
I use the word “impetus” in conversation fairly often but I always pause a split-second before saying it.
Nuclear and cellular. Both words sit weird on my tongue. I have to concentrate really hard to say them properly or else I get to enjoy people saying things like “haha! you said new-clur”
Frustrate. I can never remember if it’s “frustrate” or “fustrate.”
There’s a town halfway between Greenville and Spartanburg in South Carolina whose name is Greer. But pronouncing it the way it’s spelled marks you as a newbie. Folks who have been around for a while know it’s pronounced Grrrr.