Update: This seems to be fixed in the newest (as of this writing, 5.3.1f1) version of Unity.

If you use Unity3D and have experienced crashes launching up MonoDevelop, you may need to be an admin user. Or, if you are non-admin user, you may need to change the permissions to basically be writable by everyone (a horrible workaround):
Monodevelop-Unity on OSX still requires admin rights to run.