Download this file. It's a tarball containing 2 gems (ffi-rzmq and zmqmachine) and a C library that is loaded via FFI. There is also a script (install_me) that will ask for sudo permission to install the C lib to /usr/local/lib.
After running 'install_me' there will be an unpacked gem directory for zmqmachine.
% cd zmqmachine-0.7.1/examples
% jruby pub_sub.rb
This should result in a spectacular backtrace complaining about a NoMethodError for some FFI function.
Interestingly, this same code runs fine on Windows (though the JRuby master is from Friday, 20111216).
Trying to run jruby master with compile.mode=OFF and/or compile.invokedynamic=false has no effect. It still crashes though the backtraces will be different.
The line it crashes on is a call to an FFI-wrapped C function.