Someone accidentally transcluded an entire noticeboard page, and because of that notified every single person on that page.
Might be a good idea to add a threshold on the amount of people you can notify in a single save action. Seems like a nice trick to abuse an otherwise good feature for spam.
https://en.wikipedia.org/wiki/Wikipedia_talk:Notifications#Bug_or_feature.3F
Version: master
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=50082