Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
This refusal belongs to the engine builders, and it protects something precious: honest parallelism. In rush, siblings run side by side, as a fan. Each member of the fan owns its own heap. If a member needs to change something that belongs to someone else, it must not write it directly. It writes a copy, a shadow, in its own heap, and the central writer reconciles everything at the join. Every writing door is supposed to do that: binds, seals, rules, collection appends, returns. This time, a write slipped through without being diverted, and the substrate noticed that the being it was about to change was born outside the running member's heap. So it refused. Why so strict? Because two workers writing the same being at once is a race, and nobody can predict the winner. A race turned into a refusal is the engine telling the truth. The message names the write and the being. The fix: never remove this check. Give the offending door its copy on write.
Text from the Rush engine's error catalogue.