Android is a complex, multifaceted beast with a lot of moving parts that can break. In the grand scheme of things, this bug isn't a huge deal. It doesn't affect functionality, but it sure is annoying. Here's the deal: get an expanded notification with more than 7 elements on Android 5.1, and you'll probably see a line at the bottom that says, "@17041057."
This bug has been reported on the Nexus 4 and Nexus 5 via the issue tracker, but I've personally seen it on the Nexus 6 and now on the Nexus 9 with Android 5.1.1. According to one of the users in the Google Code thread, this is a notification or resource ID that Android mistakenly sticks into long message lists. It can apparently happen with any app that uses the standard Android notification service.
All the reports right now are from Nexus devices, but there's almost nothing else running 5.1 to test. The 2014 Moto X 5.1 update had the bug as of the last soak test, but I'm not sure about the final rollout. Google hasn't officially acknowledged the issue, but it's minor enough it might just be quietly fixed at some point in a future update.
- Google Code