RTFM or better don't...
While I am writing about curious bugs, here is another one, although technically it is not really a bug.
When setting up Icinga with mod_gearman, I wondered why service-checks where running on the assigned mod_gearman worker node, but host-checks were running on the main Icinga server and were not distributed using mod_gearman. I checked the configuration again and again, but could not find an error. Also searching the web did not bring much useful information.
The only thing that I could find were hints that do_hostchecks had to set to "yes" in /etc/mod-gearman/module.conf. But according to the mod_gearman documentation, this option was set to "yes" by default.
Well, as it turns out, the flag is set to "no" by default, at least in the version of mod_gearman that is available in the software repositories of Ubuntu 12.04 LTS (Precise Pangolin). By the way, the manual that is distributed in the source archive of mod_gearman 1.2.2 (this is the same version that comes with Ubuntu) says the same, so it is not a thing that was changed recently.