I think you are thinking about a different kind of overflow (perhaps having a column-span element which itself is wider than multicol box?). The case mentioned doesn't have any overlap, it is column-span element that just doesn't fit and ends up in an overflow column. See the picture in the first email of the thread.
From: Brad Kemper [mailto:brad.kemper@gmail.com]
Sent: Wednesday, August 25, 2010 6:54 AM
To: Alex Mogilevsky
Cc: robert@ocallahan.org; Håkon Wium Lie; www-style@w3.org; Chris Jones
Subject: Re: [css3-multicol] column balancing and fixed height (RE: Question on Flexbox balance implementation)
On Aug 25, 2010, at 1:07 AM, Alex Mogilevsky wrote:
Noted special case:
If an element with "column-span:all" does not fit into multicol box, it is pushed to overflow and its 'column-span' property no longer has effect. There is a possibility that without "column-span:all" it would have fit within the last column of the multicol box, but that possibility is not evaluated. The element remains in overflow uncioniditionally.
I imagine one could 'float:left' the column-span element in order to keep its overflow from overlapping with the overflowed contents of the columns, right?