A mixed metaphor occurs when you begin by comparing something to one thing and then shift and compare it to something else entirely. (Ultimate Style: The Rules of Writing 28 Oct. 2007; http://www.sparknotes.com/writing/style/topic_127.html)

Jack Lynch, Guide to Grammar and Style 28 Oct. 2007; http://andromeda.rutgers.edu/~jlynch/Writing/m.html

Mixed Metaphor. In a metaphor, one thing is likened to another — whether my love to a red, red rose, or the thing that supports a tabletop to a leg (as in "leg of a table"). . . .

A vivid metaphorical imagination is a sign of a good writer; a bad one is a sign of a bad writer. Here's the danger: it's possible to use metaphors badly without knowing you're using metaphors at all, because they're far more common than we realize. The secret is to pay attention to those between living and dead (we might call them "moribund"). If we forget that they're metaphors, they can become hopelessly scrambled. Consider this sentence, a more or less realistic example of business writing:

We were swamped with a shocking barrage of work, and the extra burden had a clear impact on our workflow.

Let's count the metaphors: we have images of a marsh (swamped), electrocution or striking (shocking), a military assault (barrage), weight (burden), translucency (clear), a physical impression (impact), and a river (flow), all in a mere twenty words. If you can summon up a coherent mental image including all these elements, your imagination's far superior to mine.

That was a made-up example; here's a real one, from The New York Times, 11 June 2001:

Over all, many experts conclude, advanced climate research in the United States is fragmented among an alphabet soup of agencies, strained by inadequate computing power and starved for the basic measurements of real-world conditions that are needed to improve simulations.

Let's see: research is fragmented among soup (among?); it is strained (you can strain soup, I suppose, but I'm unsure how to strain research); and it is starved — not enough soup, I guess. Or maybe the soup has been strained too thoroughly, leaving just the broth. I dunno.

It's not just journalists who make blunders like this. Joseph Addison, one of the most important writers of the eighteenth century, included these lines in his Letter from Italy:

I bridle in my struggling Muse with pain,
       That longs to launch into a nobler strain.

Samuel Johnson took him to task in his Lives of the Poets:

To bridle a goddess is no very delicate idea: but why must she be bridled? because she longs to launch; an act which was never hindered by a bridle: and whither will she launch? into a nobler strain. She is in the first line a horse, in the second a boat; and the care of the poet is to keep his horse or his boat from singing.

This doesn't mean metaphors can never be mixed. Sometimes they're good for comic effect. Sometimes they make for vivid characterization: Hamlet has a famous one, when he considers whether he should "take arms against a sea of troubles" — arm yourself all you like; the sea doesn't care — but it's dramatically effective. Most of the time, though, mixed metaphors show a writer not in control of his or her material.

The moral of the story: pay attention to the literal meaning of figures of speech and your writing will come alive. . . . [Revised 11 June 2001; revised 21 Dec. 2004.]

UVic Writer’s Guide 28 Oct. 2007; http://web.uvic.ca/wguide/Pages/SentMixMet.html

A mixed metaphor attempts to create an extended comparison but fails because it is not consistent with itself. For example, in an essay on the language used in describing pain relief medicine, a student wrote:

"The topic of pain relievers seems clouded in a sea of medical terminology."

The metaphor is mixed because the images of cloud and sea do not match. The student should have said either "drowned in a sea of medical terminology" or "clouded in a fog of medical terminology." Metaphor can be effective, but do not put too much weight on your own ingenuity; it might collapse under the strain.

“Mixed Metaphors...to delight and amaze you” (Calvin College English Dept. 28 Oct. 2007; http://www.calvin.edu/academic/engl/lang/mixmet.htm)

Once you open a can of worms, they always come home to roost. (Mike McConnel on WLW Radio, Cincinnati - contributed by Bruce Haining)

With Lenny in, Carl will fold like a domino! (The Simpsons, contributed by Michael Skinner)

Announcer during 1998 Valparaiso vs. Rhode Island basketball game: "We'll just have to see if they can keep this Cinderella slipper alive." (Contributed by Susan Bruxvoort Lipscomb)

We'll tackle that bridge when we come to it. (Colorado Springs Gazette, quoting former Sky Sox manager, now a major league baseball manager; contributed by Doug James)

You're out on a limb without a paddle. (Robert V. Dodd)

Philip Freneau had one foot in the 18th century while with the other he hailed the dawn of a new day." (Glenn Meeter's student)

If you spill the beans, then you'll open a can of worms

I'd like to be sitting in his shoes. (Doug James's student)