Sorry about the low-quality of this bug report. I can't reproduce on demand, but will keep an eye out for it.
I logged in on an iPhone, and received a light blue notification banner at the top informing me that I had received a Nice Answer badge. I dismissed it and continued browsing.
A short while later, another light blue notification banner appears, with some wording I have forgotten, but which basically said 'There's an additional notification to display." I thought that was odd - rather than tell me that there is another notification, why not just show it? But I selected it to see the notification, and it disappeared.
It popped up again shortly afterwards, and I repeated the experience.
I think it may have happened a third time, and as I dismissed it, I thought "Odd. Next time it happens, I should take a screenshot and report the bug."
Naturally, it didn't happen again.
Logging in with a real browser, I can't see anything in my history that might have triggered a notification except the single badge.