Harbrace 23c(1): Do not mix metaphors.
A metaphor is a figure of speech in which two persons or things unlike in most respects are compared by implication; i.e., without the use of "like" or "as."
As in: He is the black sheep of the family.
A mixed metaphor combines different images, creating an illogical comparison.
As in: You've started up the rocky road of algebra. Now you must wade through it.
(If the comparison uses "like" or "as," then it's a simile.)
The Dilbert Newsletter collects mixed metaphors and other malapropisms from office life for our enjoyment. Here are a bunch:
"Sometimes you shoot your foot off to spite your face."
"If it can't be done today, don't wait until tomorrow."
"I'm sweating like a bullet."
"Monday morning the fan is going to hit the roof."
"We're having this meeting to make sure all our ducks are on the same page."
"Yeah, I've got a lot of black sheep in my closet."
"You don't want to put all your legs under one blanket."
"Call me back at your least convenience."
"It's six of one and one of the other."
"I can't do it in the spur of a hat."
"I don’t want to run any flags up the telephone pole."
"You know I’m just pulling your lamb?"
"I've been running around like a chicken with my legs cut off."
"This has been a red herring around our necks."
"Like water through a duck's ass."
"We're treading on thin water here."
"Knock it off before I beat you with a dead horse!!!"
"I don't want to go out on a limb and shoot myself in the foot."
"I've just got my feet in too many pies right now."
"Gee, we haven't been here since the last time."
"'I see,' said the blind man to the fly."
"You can argue until your eyeteeth turn blue in the face."
"I swear on my dog's breakfast!"
"This library attracts deaf patrons like a siren's song!"
"Don't bite the mouth that feeds you."
"I keep telling you these things, but you keep turning a blind cheek to it."
"Screaming like a chicken with its head cut off."
"I hope I haven't used a sledgehammer to teach my grandmother to suck eggs."
"She was born with a silver spoon up her ass."
"This guy's sharp as a cookie."
"I beat it like a red-headed mule!!"
"Now, I do not want to toot my own wagon."
"I think you hit the nose right on the head."