I have found greens on the Rabbit Welfare Association list as being safe to feed, but I have also heard people say that they aren't safe.
From the above link, select the 'leaflet' 'Going Green' and there is a big list there- it's a PDF document!
I have fed green beans (the fine bean kind) to my buns before, very sparingly, with no ill effects. They don't get them at all regularly though, and haven't for a while. I believe that they are quite sugary and therefore likely to cause gas, which might explain where the view that they are unsafe comes from? I'm not sure...