We've got MediaWiki 1.13.2 and we'll soon be upgrading to the latest & greatest version (probably 1.16). We've got lots of extensions installed (for which we'll probably also need to get the latest versions) and we've done some minor configuring (e.g. adding new edit buttons).
What should we watch out for during the upgrade? And what should we test? Does anyone have any experience of things that went wrong?
If you have a lot of extensions installed, there's no way to tell for sure if anything will break. I suggest you make a copy of your wiki installation to another machine and perform an experimental upgrade there. You can try to upgrade to 1.16.0beta2. Based on how that goes, you'll be able to decide when to do a real upgrade.
As a general rule, you should also consider the Upgrade section that MediaWiki includes in all their release notes. In 1.16.0beta2, it says this:
== Upgrading ==
1.16 has several database changes since 1.15, and will not work without schema updates.
If upgrading from before 1.11, and you are using a wiki as a commons reposito- ry, make sure that it is updated as well. Otherwise, errors may arise due to database schema changes.
If upgrading from before 1.7, you may want to run refreshLinks.php to ensure new database fields are filled with data.
If you are upgrading from MediaWiki 1.4.x or earlier, some major database changes are made, and there is a slightly higher chance that things could break. Don't forget to always back up your database before upgrading!
See the file UPGRADE for more detailed upgrade instructions.
Finally, you have to realize that official releases may not be completely compliant with the previous beta releases, so consider doing an experimental upgrade with the official 1.16 version when that comes out. If you don't want to wait for 1.16 you may upgrade to the latest official release, i.e. 1.15.3