Oh yeah. Cinnamon is cimmumumumumum.
We gave our cousin a new nickname by pronouncing her name wrong. Pris-sky-la (Priscilla) becomes Sky. Better than her previous nicknames - Prissy and SillyHead.
Susan
Tell me more!
We just make up shit at work on an almost daily basis:
approximatize, exactosity, correctitude, fuzzfactorated, errorness, wrongosity…
but we all seem to know what we are talking about.
My friends from Tenn usually say
Jeet yet?
No, jew?
I say “nanner” for banana, cinnaminamin, laundrymat…
Used to be I would make a peanut butter sangwich. Now, of course, I make a sammich.
I also pronounce Missouri like a native resident: Missourah.
Thanks to Bernie Mac, I will occasionally call someone a summummabitch.
We’ve lost x’s in favor of s’s in my household: Was that blouse “espensive”? Did you buy anything “estra” whille there? Hope you don’t need to get your teeth “S-rayed”.
When my much younger brother was a toddler, he called elephants “ele-sips”. I’ve kept the tradition up, even tho I suspect he doesn’t recall it at all.
–Beck
Peatot. It’s what you make tea in.
Graduatization. Gradgimafication. Gratiguation. Gradumatation. This started as a superstition in my last year of college, for some reason I felt like I couldn’t say the actual word until it was really, absolutely positively going to happen. Then I couldn’t stop.
Jootz. My friend’s two year old really likes to drink Jootz.
I have lived in Missouri my entire life and it’s pronounced Missouree. As in, Misery loves company. Only grizzled old men who smoke unfiltered Pall Malls while combining the corn field pronounce it Missourah.
I also say sammich. Elephants are elly-fonts. Puppy dogs are poopy dogs. Mom’s cedar chest is the sonofabitch. Although that’s more a renaming than a mispronunciation, really.
I had a boyfriend who couldn’t say ‘relevant’.
I found ‘revelant’ sort of charming, and also a nice way of playing with ‘reveal’, so I kept it. 
[ul]
[li]Pizza- I sometimes say it as it appears, not “Pete-sah”.[/li][li]I sometimes substitute an “oi” sound for the “er” sound in words like “turn”. I’ll say “toin”. or “Ow! That hoits!”[/li][li]A friend of mine calls hamburgers “hanga-burgers.”[/li][li]I also read French words as they would be pronounced in English. “Faux pas” is “fox paws.”[/li][li]I also run initialisms (as apposed to acronyms) together. I say “at’em” when I mean ATM, for example.[/li][/ul]
From my daughter, when she was 3 or 4; ‘elfetent’ for elephant, ‘fat tood’ for cat food. I can’t remember the last time I actually said ‘cat food’.
‘Whasdisheah sauce’ for worcestershire sauce; ‘stragedy’ for strategy; ‘bookay’ for bucket (thanks Hyacinth!); libarry; sammich (sammidge, sanguidge); fox pass; axe (‘ask’); axterix (asterisk); sommers (somewhere); excetra, excetra…
My roomate in college 25 years ago used to say Befkist for Breakfast. I still find myself doing it occasionally.
Calcubator for calculator, as if it were hatching the numbers.
Mexico is Mestico sometimes.
Tostitos chips are toasty toes.
Kabul is the capital of Half-and-again-Stan.
Cabbitch for cabbage.
Khufu’s Pendulum and the Great Pyramids of Foucault.
Paucity is spoken slowly as Paw City.
Around here we change channels with the Rote McTroll.
We like syrup on our warfles.
I recall a headline from the Onion a few years ago: “African-American Community Terrorized by Ask Murderer.”
I’d give the award to ‘pooptaste’!
All this time I thought the wrong way was the right!
I mispronouce things to help me remember how to spell them. Like, definite becomes de finite!
From the produce aisle:
taters
nanners
pin-uh-PULLS
I say bee-uh-YOU-ti-ful because it helps me remember how to spell it correctly.
From Winnie the Pooh: heffalump
Not exactly a mispronounciation, but I like to describe the weather/outside conditions using the phrase “much of a…” as in “much of a hotness,” “much of a wetness,” “much of a darkness,” etc. (from Alice in Wonderland’s “much of a muchness”).
<pokemon nerdery>
Heyyy, there’s a Pokemon called Skitty for that reason! 
</pokemon nerdery>
I’ll pronounce chihuahua as phonetically as “chi-hooah-hooah”
Another one I forgot. In my office, any word that is different in Queen’s English from American English is a “Britishishism”. In memory of another native-speaker co-worker who just couldn’t pronounce it properly, no matter how we tried to walk her through the word.