Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your script was in the middle of some arithmetic, and the number engine said it could not finish. Here is how to read that. Every number in rush is exact. It is a decimal or a ratio, computed by a part of the engine called tungsten, using a small workspace lent to each step. Usually that is plenty. But sometimes a calculation grows beyond the workspace, say a power that produces thousands of digits, or the result is something the engine simply cannot represent. When that happens, tungsten reports a failure, and the step has no number to move forward with. Rush could round, or guess, or return zero, but any of those would be a quiet wrong answer. So it stops, and names the operation that failed in the second slot of the message. Picture raising a number to a huge power inside a loop. The fix is to look at the operation named and ask whether the numbers involved are bigger than they need to be. Break the calculation into smaller parts, or limit the size of your inputs. Exactness is the promise, so rush will stop before it breaks that promise.
Text from the Rush engine's error catalogue.