When preparing ourselves for a difficult challenge I offered the opinion to my team, “We’ve got to grab the bull by the horns and wade through this thing.”
Mercifully no one called me on the mixed metaphor, but I’ve never forgiven myself.
I’m seeking your help to compile the definative list of mangled metaphors, which I will transcribe by hand onto toilet paper as penance.
For starters:
“Surf the web”
“Cold as hell”
“Having to eat the elephant is a hard pill to swallow.”
My favorite is “colder than a witch’s brass monkey”. I can’t remember where I heard it first, but I use this one all the time.
“That’s the way the kitchen sinks.”
There was a line on West Wing that just tickles me. Josh is talking to Amy, who just went to the First Lady about an issue that Josh had earlier dismissed.
He says, “You went above my head behind my back,” to which Amy replies, “Quite the contortionist am I.”
I love Aaron Sorkin.
It’s not so much a mixed metaphor as a mangled simile, but what I thought of on seeing the OP was the universal retort for emphasis:
Does the pope shit in the woods?
Being a lifelong procrastinator, I sometimes tell myself, “Keep fucking the dog and you’re gonna screw the pooch.”
These two, outwardly similar metaphors actually don’t mean the same thing at all. Fucking the dog is dragging your heels, dawdling or wasting time. Screwing the pooch is messing up a task or blowing an opportunity, usually used in the past tense.
When I was about 9 my Dad, brothers and I were playing a little 2-on-2 football in the front yard. I’m on defense guarding the QB, and I yell at my brother who’s covering the receiver…
“Stick to him like glue on a blanket”
Well, my Dad thought it was funny 
I thought I made this one up. At least, I’d never heard it before I said it. A search results in a couple-thousand hits, though. Anyway, I’ve been saying it since the '80s:
We’ll burn that bridge when we come to it.
As far as this metaphor goes, I always use the one attributed to Ted Kennedy…
From I Always Look Up the Word “Egregious”, “When the thief struck her, she fell down with a thug”.
That’s a kettle of fish of a different color.
You’ve buttered your bread, now lie in it.
“There’s Ubangis in the fuel supply!” A good catch-all for any kind of weird problem.
If the shoe fits, wear it on the other foot.
Not really mangled, but changed by Heinlein: No sense closing the barn door after the barn’s been stolen.
This lyric from “Little Mary Sunshine”:
When asked to answer an unswerable question by a patient, like “when will it stop hurting completely?” I like to fall back on this little soliloquy: “How high is up? What is the sound of one hand clapping? Is the pope catholic? Some things we just don’t know, you see.”
Second favorite response: “When the pain is gone, it won’t be there anymore”.
“Look folks, this isn’t rocket surgery.”
One brick short of half a deck of cards in the water.
You can lead a gifthorse to water, but you can’t look into his mouth.
“Well, that’ll throw a monkey in the wrench!”
“let it roll off your back like a duck”