On Jan 6th around midnight UTC and 10:30 UTC, Gerrit had some troubles reaching its ReviewDB (apparently on m2-master.eqiad.wmnet ).
I am not sure how serious it is, the web interface spurts out an error message such as:
Guice provision errors: 1) Cannot open ReviewDb
A side effect is that Zuul ends up stalled if it is attempting to add a comment to a change while the DB is not reachable ( https://wikitech.wikimedia.org/wiki/Incident_documentation/20150106-Zuul ).