A correspondent from Brazil cites the sentence 'It was clear that the germs of a compromise policy would have to be sown', commenting: 'Such mixed metaphors can easily slip out in unplanned English, but in careful writing you must be on guard against them. Replacing germs by seeds would repair the metaphor. He then adds: 'I would interpret such sentence as evidence of users´ linguistic creativity', and wonders if I would agree.
Mixed metaphors have certainly had a mixed press. Fowler was strongly against them, calling them an 'offence', and getting especially angry when people asked permission to mix - 'if I may mix my metaphors...'! Some of his quotations are indeed highly incongruous or semantically obscure, and that is when the stylists have a point. But most everyday mixing is not like this. In speech, I doubt whether anyone would notice the 'germs' example, because the close semantic connection between germs and seeds conveys the user's intention well enough. Nor in speech do people really have trouble handling famous examples (i.e. often quoted in style books) like 'He had his head so deep in the sand that he didn't know which side of the fence he was on'.
Note that it's the juxtaposition in a single sentence construction here which causes the judgement of mixing. If the utterance had been 'He had his head deep in the sand. He didn't know which side of the fence he was on', we would talk instead of a sequence of metaphors. We might or might not like the sequence, but that's a different point.
Stylists don't like mixing in formal writing, where they prefer people to have taken more care in their thinking. But a blanket condemnation of mixed metaphors in writing is absurd - for if one does so, one ends up banning a great deal of Shakespeare. Dozens of examples come to mind. Here are just three:
'Was the hope drunk / Wherein you dressed yourself?' (Macbeth)
'all the voyage of their life/Is bound in shallows and in miseries' (Julius Caesar)
and, most famously,
'To take arms against a sea of troubles,/And by opposing end them' (Hamlet)
If this isn't linguistic creativity, I don't know what is.