Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
This one is not the engine complaining. It is you, or the author of the script, deciding to stop. Panic is a builtin that halts the run, with a message written in the author's own words. It is not a failure of the engine, and it is not a wrong answer. It is a declared stop. Think of pulling the emergency brake on purpose, with a note taped to the lever explaining why. Because it is an ordinary refusal, you can catch it. Wrap the risky part in a try, and the catch arm receives the rendered message, so the script can decide what to do next. If nobody catches it, the engine runs nothing after the panic, and the message is shown as written. So when you see this, read the text after the colon. It was written by a person, to tell you what they considered impossible or unsafe at that point. Fix whatever condition triggered it, or catch it if stopping was not really needed. Panic is a clear voice, not a mystery.
Text from the Rush engine's error catalogue.