SyntaxError: Bad control character in string literal in JSON at position 16 (line 1 column 17)

## Error

```
Bad control character in string literal in JSON at position 16 (line 1 column 17)
```

## Stack trace

```
in JSON.parse
../node_modules/@rails/actioncable/app/assets/javascripts/action_cable.js:278 in Connection.prototype.eventsmessage
```

## Tracking Details
- **Project**: exercism-js
- **Sentry**: [SyntaxError: Bad control character in string literal in JSON at position 16 (line 1 column 17)](https://thalamus-ai.sentry.io/issues/141490566/)
- **Culprit**: JSON.parse()
- **First seen:** Aug 19, 2026

Start with the Sentry report and the stack trace at ../node_modules/@rails/actioncable/app/assets/javascripts/action_cable.js:278, focusing on the JSON.parse call handling the exercism-js connection message. The issue does not name a source file or test; done means identifying the malformed message source and preventing this parsing error from recurring.

Written by the indexing model from the issue text.