If a notification has symbols in the title, then, all notifications will disappear, and you have to wait 48 hours for the notification to expire, or find the source (if the notification can be removed by viewing the source of it)
You can try it by commenting on this blog and waiting for a reply from me.
I've tried this on DSi with DSiPaint only, but as far as I know, it's common on other platforms.
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