Let's begin with the idea of a dead metaphor. As a kid in Catholic school I was taught that limbo was where dead, unbaptized babies went until God's final judgement. Of course these days the word limbo is used to refer to a waiting period before a resolution. For example, Congress declined to act on the bill, consigning it to limbo until the next session. The word limbo is a dead metaphor, because it's original religious-context meaning has been interred, so to speak. Now let's look at what can happen to a dead metaphor.
Remember when one of the Boston Marathon bombers was killed? It took a week before his body was accepted by a cemetery. NPR reported that during that week his body lay in limbo. The reporter used the word "limbo" in its contemporary sense, as a place of waiting, but unintentionally echoed its original meaning. When I first heard that news report I thought, so if a dead metaphor comes out of its grave and walks it's no longer a dead metaphor, but it's not alive either. What is it? My answer: It's a zombie metaphor.