My grandfather always used to say, “GOT…to be more careful!” Even as a little kid, I knew what he was working hard not to say.
Son of a BUN!
Holy Frito-Lay!
And the ever-popular: jinkies!
Sommanumbatch!
You bastige!
Fargin’ icehole!
Lousy corksucker!
I’m a keek you in da bells!
First time I saw The Exorcist on TV: ‘Your mother still rots in Hell!’
I love that commercial. It’s the only one that I’ll stop fast-forwarding the DVR to watch. No one but my wife gets it when I say that, though.
I feel like there was a different version of that commercial, though. Am I imagining that?
Hully gee! Don’t nobody remember the Yellow Kid no more? 
Aw, phooey!
It’s been waaayyy too long since I’ve seen Johnny Dangerously! 
It was either SNL or MAD Magazine that used “Your mother sews socks that smell!”
Back during the heyday of silly politically correct language, I recall an attempt to get people to call the old “seasoned humans”.
Mmmm, seasoned humans. Medium rare. NOM NOM NOM
Speaking over overdubs, I’ve always been partial to “Yippie-kay-yay, mister falcon” and “This is what happens when you meet a stranger in the Alps.”
That ditzy teen from The Music Man: “Ye Gods!”
Q
Example of a reverse euphemism, (msimehpue?) i.e., the euphemism is worse than the phrase it replaced: “fart sack” for navy mattress cover.
“fart sack”, meet “ass hat”. 
Q
“This is what happens when you feed a soldier scrambled eggs!”
Also, Spike Lee used “melonfarmer” for “motherfucker” in at least one of his movies’ cable dubs.
Got the first one, SFG, but if the second one was supposed to be, “… fuck a stranger in the ass”, then I think I could have done a LOT better overdubbing to something else!
Q
Yup! You nailed it. IIRC, they actually did the edited dubs on The Big Lebowski by just watching it and coming up with words that kind of fit the mouth movements. (I think I might have mis-rememberd: it may actually be “find a stranger,” not “meet a stranger.”)
Another from W.C. Fields, “Mother of Pearl!”
Motor trucker
Oedipus. Usually a fella will look blank for a second when you call him Oedipus.
Jesus Ramirez
Movers and shakers
Flithering gheen
Backtracker
Maybe that politician had started to say something else when he said Macaca.
Fox and hounds
Some brands names make good expletives:
Filter Queen
Blodgett
Square D switch plate
If you can gather your wits, you can lean in close to say, “You’re a fine fellow, and your mother is proud of ya!” in a strong declaritive voice.
Maybe, but I hardly think calling a dark-skinned man a ‘monkey’ in any language can be rightly considered a euphemism.