Native-English speakers sometimes make style errors that make understanding English more difficult. One of these is the mixed metaphor. A metaphor is a word or phrase that means one thing, but is used to describe another thing to show their similarities in an interesting way. For example, "Don't make a pig of yourself." means Do not eat too much. or Do not act like a pig.
A mixed metaphor is when the speaker mixes two unrelated metaphors. For example here are two common metaphors that use sports and games as imagery: 1) "Step up to the plate" describes baseball when it is your turn and that means that one needs to do something. 2) "Lay your cards on the table" describes a card game where it is time to show if you can win or not and means to speak honestly. A mixture of these metaphors might be, 'step up to the plate and lay your cards on the table.' Now the imagery does not make much sense. Which game is being played? What is being asked of you?
Between people who are very familiar with both metaphors these combinations can be fun. However since they usually destroy the imagery of both the metaphors, mixed metaphors are poor English style. They should be avoided when talking with anyone who might not fully understand both metaphors. This is especially true for native-English speakers communicating with second-language speakers.
Sometimes playing with mixed metaphors can be a fun way to create a poetic image. For example, 'A rolling stone heart gathers no moss' combines two metaphors 1) “A rolling stone gathers no moss” meaning people who are always moving and changing (ex: jobs) avoid responsibilities. 2) “heart of stone” meaning to have no feelings, especially sympathy or pity. The combination means that people who are always moving are uncaring. It is possible to have a mixed metaphor that maintains sensible imagery. In this case both use a stone as an image. Generally both metaphors must share an object and have related meanings in order to be a functional mixture.
This topic raises some questions. Is using a mixed metaphor, even with a consistent image, good English? Is any mixed metaphor only a poor style choice or an error? Since a mixed metaphor makes the sentence very difficult to understand it is an error. Errors can be used for artistic expression, such as in poems. For any other use, it is an error, even when the speaker is a native.