I'm trying a very simple script, that leads to core dump on Mac OS:

var ffi = require("ffi");
global.gc();
console.log("END");

When I call it with:

iojs --expose_gc --nouse_idle_notification --trace_gc test.js

I receive core dump.

I've set DEBUG environment variable and you can find the output in this gist I've also placed there the core dump file in case it can help.
I'm using iojs 3.3.0 and latest versions of ffi and ref:

 iojs
> process.versions
{ http_parser: '2.5.0',
  node: '3.3.0',
  v8: '4.4.63.30',
  uv: '1.7.3',
  zlib: '1.2.8',
  ares: '1.10.1-DEV',
  modules: '45',
  openssl: '1.0.2d' }

Please note the crash is on the line global.gc().

I've tested this with iojs 3.3.0, node 4.0.0 and the result is the same.