I am now co-editor of Multi-column Layout. As the last publication of the CR was in 2011, there is some history, and a whole host of interop issues. I'm digging it all up.
I am curently searching through the archives of www-style to surface any resolutions and discussion relating to the spec, in order that I can turn them into issues on GitHub, and also create a Disposition of Comments for the CR.
People have started sending me their multicol woes. Most of these fall into things that are likely to end up in the next level of the spec or are interop issues against the current spec, or how it interacts with other layout methods. I don't want to lose track of them however. So, I suggest that if you have a multicol woe, interop issue, or some suggestion raise an issue here.
If it is an obvious spec issue then you can raise it directly over on CSS WG Drafts. You can also log browser issues directly with the browser concerned. However I'm aware that a lot of the stuff is not that neat and tidy - hence this list!