This is a usability issue, encountered with FEATURES=collision-protect.
I just ran "emerge -uavND world" and got back to find the screen thus:
* GNU info directory index is up-to-date.
What had happened was that sys-apps/man-pages-posix had produced quite a few file collisions. In fact the list was so long that it filled my whole scrollback buffer. I had no way to get at the actual error message to know what all these file names were about. "emerge man-pages-posix 2&>1 | less" did help me there, but I wouldn't expect the average user to think of this.
Ideas for a solution:
1. Truncate list to a sensible number of files, displaying a big fat warning that there are yet more files. Might cause problems when all displayed files belong to one package but files belonging to a different package are not displayed. Writing the full list to a file and giving its name along with the list might be a viable solution here.
2. Print some useful information at the end of the list. One might even swap the list of collisions with the message about what a collision is and how to deal with it.
What I'd like to do is show an addition message at the bottom if the number of files exceeds a certain number. That way the user should have everything they need to know.
This is fixed in 2.2_rc4.