The easiest time to add insult to injury is when you're signing someone's cast.
Signing a Cast
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Explanation
Experiment
Explain Bear
Listen up, buttercup. This joke is all about a play on words. "Adding insult to injury" is a common saying about making a bad situation worse. But here, it's literal because you're writing an insult on someone's broken bone cast. Get it now, or do I need to draw you a picture?