Today I upgraded the slave databases to a new major release of the database engine. I'd like to use some of the new features to increase the performance of the system.
The upgrade itself, and the necessary conversions, wouldn't have caused any outages, since I can tell my software to do queries on another database server while upgrading one (in fact, they will automatically fail over to another server if one is down). But, as usual, there was something I overlooked, and for a few minutes about 50% of the /info/ page requests failed and complained about problems with looking up nearby cities. I had to improve the stored procedures a bit to get them working on the new database engine version.
Please, drop a note in the blog comments, if you notice any other problems.