Why does everyone want me to create an alert for severity 025? Brent Ozar and others suggest to listen to severity 025 SQL-Server events. I searched for the meaning of all different high severity events, looked through different articles and read the most recent msdn documentation. But the only severity that seems to be monitored by everyone but explained nowhere is 25. 

What is severity 25, where is it explained and what does it mean?
Severity 25 is a 'catch-all' fatal system error.


  A severity 25 error is a fatal system error. I have heard that
  severity 25 is more or less a catch-all for miscellaneous fatal
  errors. I have only seen this error when related to failed upgrades:
  something prevents one of the upgrade scripts from running, and a
  severity 25 error is thrown


http://sqlperformance.com/2015/04/sql-performance/dealing-with-high-severity-errors-in-sql-server