At least all of those as stupid as I am. When developing cintoo Messages I got biten (again!) by the MacOS filesystem not being case sensitive. All tests worked fine but on the integration server (thanks pulse guys for providing your software, it helps) which is running on Linux the tests wouldn't work. My bundle files were called test_de_de.properties which worked on MacOS X but not on Linux, where they have to be called test_de_DE.properties. Well well, write once ... 😉
Of course I recognized this after a long debugging session which even drove me to download the JDK source. Sigh.