> For the complete documentation index, see [llms.txt](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/messaging-troubleshooting/a-missing-event-among-billions-how-we-found-it-on-an-event-mesh/day-12-13-use-case-7-unexpected-consumer.md).
# Day 12/13 Use Case 7: Unexpected Consumer
**The event on the same queue is subscribed by an unexpected consumer.**
This issue consistently occurs after a major release because outdated applications are still running and subscribing to events. As a result, these applications reject the request event message due to parameter errors. The outdated applications result from rolling upgrades across multiple sites and hundreds of instances. To adhere to tight release windows, these applications need to be upgraded in multiple phases and sites. Surprisingly, we have encountered this โghostโ process issue a few times on different systems in the last two months.
To resolve this error due to payload format change, simply change the version number on the topic and queue name in the new release. Then force the new application to publish events to the topic or subscribe from the new queues. Additionally, we recommend restarting the application after the upgrade to avoid any โghostโ processes lingering in the memory.
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