Fixed in the next release.
This occurred if users were subscribed to a page or discussion that later was deleted by a moderator. The code that was designed to remove subscriptions whenever the original content was deleted was not working properly (I'm surprised it wasn't throwing database errors itself).
This code has been fixed in the next release. Also, the upgrade script contains a number of steps that will cleanup the stale subscriptions from the database, so that users who were already affected will be able to manage their subscriptions again.