English professors love to catch mixed metaphors, when grading papers. Here's list from some students papers:

• He swept the rug under the carpet."
• She's burning the midnight oil at both ends."
• It was so cold last night I had to throw another blanket on the fire."
• It's time to step up to the plate and cut the mustard."
• She's robbing Peter to pay the piper."
• He's up a tree without a paddle."
• Beware my friend...you are skating on hot water."
• Keep your ear to the grindstone."
• Sometimes you've gotta stick your neck out on a limb."
• Some people sail through life on a bed of roses like a knife slicing through butter."

In the 21st chapter of John's Gospel there are two metaphors. Fishers of Men and Feed my Sheep. You could mix these metaphors and you wouldn't create a grammatical error, because Jesus has one thing in mind. It is his ministry for the church. We are to reach beyond the shores and reach outside our own fears and bring people to Christ and disciple them. I love these two images back to back - Bring them to Christ and feed them. That's our metaphor. That's our mission.