Weird Bug - Ability to see partial of other user's notification

How it was found

I was browsing through the pages and suddenly I saw this at my notification:

Screenshot: ███

The two yahoo bug notification are definitely not mine as I've not reported any bugs to Yahoo, while the Slack's report is mine. I can't really explain what happened as I do not know it myself. I seem to be able to replicate it from time to time but I don't know what is the factor that causes the bug.