Grandoise.
I use this as a form of mockery toward the manager of another advertising firm. The man (a native English speaker) is in love with big words, but is completely incapable of spelling or pronouncing them, let alone using them correctly.
He was explaining some new ad briefing and waving his arms frantically saying “It’s got to be big! Impressive!.. Grandoise!”
We had to try so hard not to laugh we almost shit ourselves.
A boss of mine from many years ago would say “so-whatever” for "whatsoever. I may have gagged a little when I heard him say it, but I never heard anybody mention it, even behind his back.
Thought of another one: I pronounce “fajitas” so that it sounds like a disease.
My mother says “sangwich” instead of “sandwich” and “prendsels” instead of “preztels”. Both of my parents say “taunt” when they really mean “taut” - like so:
Dad: “Come on, help me fold this sheet. No, pull it taunt! Pull it taunt!”
I tried correcting them many times over the years, so I can only assume that they do it on porpoise. 
Here on Merriam-Webster’s site you can find the definition . Click on the little red speaker icon to hear it pronounced. I live next door in Washington State and that’s how we all say it up here.
I’ve heard people pronounce it ore-a-GOAN, to rhyme with loan, or ore-a-GONE, to rhyme with lawn, but they’re not from around here. 
My dad’s favourites:
“Sometimes I put the em-PHAH-sis on the wrong syl-AH-ble. But it doesn’t bither me a bot.”
“Whore’s ovaries” (hors d’oeuvres).
“I’m retarded. I mean ‘retired’.”
He also used to carry a piece of paper in his wallet as a prop to his “getting older and losing my memory” schitck. “My memory is so bad that every morning I have to look at this.” He takes out the paper, which reads “Good morning. Your name is ‘Woody’. Have a nice day.” (He never did come up with an answer for how he remembered to look at the paper each morning.)
That is absitively, posilutely, the silliest thing I’ve ever heard.
Foilage.
Wooder (Philadelphian for ‘water’).
shE. Thorp and I also like to mock what we consider semiliterate pronunciations: ofTen, com-for-ta-ble, in-ter-es-ting, comPARable, preFERable, appLICable.
Ever since I saw the Monty Python skit, it’s been ‘crymanthesums’ for me.
A common surname in this area is Bullocks, and one family has a business along a road I take frequently – so I never miss a chance to shout out, ‘Bollocks!’ I dread the day I am introduced to one of them, and call them ‘Bollocks’…
I grew up only knowing the ‘alternative’ pronunciation of fuchsia, and this caused damn near hysterics in my German host family the first day I saw their wonderful garden, and said, ‘Ooooh, look at all the fuchsia!’
A childhood mispronunciation I still say, ‘antiquidated’ for antiquated. Also when I was wee, I didn’t realise the spoken word hallucination was the same as the written word hallucination, and I asked my mum what was the difference between an ‘hallucination’ and a ‘hawly-cawktion.’
Heh, from that link:
In my experience, most outsiders pronounce it as if they’re telling you Laker Robert has left the court. “Horey GONE.”
I’ve lived in California for 56 years, but I was born in Eugene, Oregon. (That’s only pronounced Oy-gaina if you’re a German, an Austrian or California’s governor.)
My dad always did this! I still do sometimes. He also taught us “juh-LAP-en-no” for jalapeno and “BYOO-tox” for buttocks. Mom always fussed at him about it because she knew that someone overhearing him was liable to believe he was ignorant, not knowing he was just being funny.
I oftentimes say “bibbit” for biscuit and “mee-mutt” for peanut because my nephew said that when he was just learning to talk and it stuck.
Here in the desert I see runny babbits (bunny rabbits) every day.
I always say ‘pacific’ for ‘specific.’ Something I picked up in the Army.
I’ve often intentionally mispronounced fillet and buffet with the t on the end.
I have a co-worker whose last name is Passage. It’s pronounced the same as the word. But when we received an email telling us he was coming, for some reason I made a joke that maybe he was French and it should be pronounced Pah-sahj (rhyming with the word massage). The joke inexplicably caught on and he now even answers to the wrong pronunciation.
Damn, yellowval beat me to it! I was going to mention my sister, who also says “croutons” as “craow-tons”. She also got this from the Cosby show episode referenced. Now my sister always says it this way out of habit. As for me, I mangle most French words on purpose and pronounce them phonetically as if to be English. I used to spoonerize “masturbate” by saying “stammerbate.” I used to drive people nuts by saying “de-lish” when I meant “delicious.” I don’t do much of this anymore, but I still like to have fun with the French words since I’m not always sure how to pronounce some of them anyway.
Must be related to my husband. Our byootox is from a movie. With Henry Fonda. On a boat.
I heard a guy, early eighties, describe a particular racist trait as “hereditatious and shit.” All within hearing laughed at him, and now we still add “–atious and shit” to everything and anything. Someone smells funny? They’re “funkilaceous and shit.” Lying? “Mendacious and shit.” Granola is very nutratious and shit. It works, honest, for everything. It’s very versatatious and shit.
Also, in Seattle there are a bunch of resale/junkstores called Value Village. Most of us call them “va-LOO vil-LAHZE.”
I call them horse ovaries. (From an old “Three Stooges”, IIRC.)
I say this a lot, too. And it reminds me how my dad, when he’d see a rabbit cross the road in front of the car, would say, “There goes a hop rabbit, jacking off across the road!”