The Wikipedia defines a metaphor as follows:
Metaphor (from the Greek: metapherin) is language that directly compares seemingly unrelated subjects. In the simplest case, this takes the form: “The [first subject] is a [second subject].” More generally, a metaphor is a rhetorical trope that describes a first subject as being or equal to a second subject in some way.
Then, the same article describes a mixed metaphor as follows:
A mixed metaphor is one that leaps from one identification to a second identification that is inconsistent with the first one. Example: “He stepped up to the plate and grabbed the bull by the horns,” where two commonly used metaphoric grounds for highlighting the concept of “taking action” are confused to create a nonsensical image.
Some times people use the language of the Old Testament in speeches to illustrate what they are trying to communicate to a group of people. Once in a while, people use the language of the Old Testament in a way that creates mixed metaphors. Take for example, the case of Mayor Dennis Donohue of Salinas, California, a city in which I lived for three years. Speaking to a group of people at their annual faith community luncheon, the Mayor told those attending the luncheon “that he sees the city as David from the Old Testament, up against the Goliath of gangs and other challenges.”
I think this is a good example of a mixed metaphor. David and Goliath are people while the city is a political entity and gangs are a group of individuals. I am quite sure the Mayor’s illustration described to his audience the great challenge the city is facing, however, in my view, he used two items that are equals (two individuals) to illustrate two items that are not equal (a city and a group of people).
Professor of Old Testament
Northern Baptist Seminary