Do you deliberately mispronounce certain words because they’re funnier that way? Do you do it just to bug someone else?
I like to say “bohemoth” instead of “behemoth.” I don’t know why, but the former just sounds more amusing to me than the latter.
“Pugnant,” for “pungent.”
My college bf said this once – 30 years later, I’m still not sure if he was whooshing me or not, but the pronunciation has stuck.
I had a professor once who liked to say “vi-ola” instead of “voila.” When and why he installed “voila” in his lexicon in the first place I can’t say.
A friend of mine says “long-er-eh” or “ling-er-eh” (both with a hard “g”) for lingerie. It used to bother me so much, but I find myself doing it all the time.
I always throw out “Hey-zus” for Jesus, and “Hor-hey” for George. No good reason.
Also, “ski-ssors” for scissors.
Cow-orker instead of co-worker. I just think it’s funny to say out loud. Cow-Orker. 
Seems obscene yet not identifiably so.
We also call my friend Siobhan (pronounced “Sha-vann”) Si-o-ba-hane, or Si-ob, or just Si. But only because she doesn’t mind. 
Shizzors (for scissors, if it wasn’t obvious).
Aluminium (I’m a closet Brit, I guess). [And that’s al-yoo-min-ee-um]
Ansyways and Anyhoo.
This put me in mind of Stephen Fry’s gag:
“He walked with a pronounced limp: L-I-M-P, pronounced, limp.”
Jebus. 
I like to add “ulate” to the end of verbs.
“I can totally understandulate” or “I’m going to do some Internet surfulation”.
In Montgomery, Alabama there was a restaurant for a thousand years called “The Elite”. Fifty years ago one of the state’s Foghorn Leghorn politicians, who really was a dumbass, called it “the ee-LIGHT”, and the name, which everybody knew was mispronounced, just stuck. By the time they closed a few years ago they had long since answered their phone “thank you for calling the ee-LIGHT Cafe” and it was just sort of a local colloquial joke, and that’s one of my favorite mispronunciations.
Unfortunately you have absolutely no idea how many guests came into Montgomery from all over the nation and delighted in pointing out to the quaint save locals that “that word is pronounced e-LEET”, never grasping that “Yes, just because we were born south of Yonkers don’t mean we ain’t be got us no culturie and sophisticitationry.”
I like to say krauten for crouton. I have no idea where I got it from, but I’ve been saying it that way for as long as I can remember.
And I sometimes say babin-suit for bathing suit. Like a babe in a suit, babinsuit. Ha, ok I’m weird.
My husband also says Vio-la, but it was from an old Fishbar animation. I say “voila” now and then, however, it is in my lexicon, because I am half French, and my great aunt liked to use it a lot when she babysat my brother and I. So, naturally, everytime I say it, my husband repeats “Vio-la!” (veeo-la!)
I picked up a few from my husband, such as instead of saying “warm”, we say worm (sounds more like werm, though). I just got back from the cinema with my husband, and when I sat on the very warm seat, just exclaimed “worms! worms! The seat is worms!”
I pluralise singulars.
I singularise plurals.
There is no rhyme or reason to these two, I just throw it in where I feel like. “Do you like a present?” “I like a pillow.” “I need a drink of waters.” “I’ll go get the cameras.” When my husband is out with me in public and when get home, he sometimes tells me in his own way he was feeling horny but couldn’t do anything about it until we got home. He likes to say he was “feeling his oats.” So sometimes if I think he’s got a hard on in public, and we get home, I ask: “Did you feel an oat?”
Tummy=tumbly
Elbow=belbow
Fingers=finners
Hello=hellos
Kitten=kidden
Mostly, they’re just being playful. I don’t speak that way in public. I talk like that to my brother, a couple of close friends, and my husband. In front of my parents or in-laws? Nooooos!
Em-bare-assing.
Tubamaba.
I also like saxamaphone and oboemaboe.
The only one that comes to mind is yuzz for you all.
There are 2 other common words I simply refuse to let pass my lips, because they sound awful when I speak them: Cookie (where I’ll use the variety as opposed to the general term) and gift certificate (where I’ll ask a merchant for a ‘gift voucher’).
Edumacate
Chimbley
Lieberry
-osity on the end of things as an intensifier: bad-osity, good-osity, drive-osity, computer-osity, etc-osity, etc-osity
Chihuahua is fun to say as Chya hoo ah hoo ah.
Aspergrass
Tucson turns into Tooque sawn.
Bee yoot i muss for beautiful.
My dad loves doing these, and used to teach me them as a kid (which was a bit mean, as it led to embarrassing situations later), but I still use many of his:
- Viola!
- Vanilla folder
- Lesbian restaurant (middle eastern food)
- Beresk
I also like A-rab and EYE-talian, and of course the SDMB has given me ‘cow-orker’, and I often substitute ‘Og’ for "God now.
My sister and I also often speak in Blues Bros accents (which is good fun for really mundane stuff):
“You want I make a cup of tea?” “Yup”
For spray paint: spraint
and the proper pro-noun-sation of words