Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You read a name, and the thing it pointed to was already gone. Here is how it happened. You dropped a holder with data drop. Say the holder was called xs. Dropping it destroys the holder itself. But another name, like ys, was pointing at the same holder. That name still exists, but now it points to nothing the engine's gate will open. So when you read ys, the engine refuses, and says by name that it refers to a dropped holder. The old behavior was much worse. It would print an empty line and exit with zero, as if an empty text were a real value. That empty text was never held by anyone, so the output was a fiction, wearing the costume of success. Now you get the truth. The message shows the name as written and why it refers to nothing. The fix: do not read a name after its holder has been dropped. If you need the data later, copy it before dropping, or keep a name that you know is alive.
Text from the Rush engine's error catalogue.