> Files that were previously in /usr/bin or in /bin can now be found in EITHER of these locations, since one symlinks the other. So no previous expectation was really broken.

I don't know, I just hit breakage the other day. I have /usr/bin before /usr in my path (which is the default on Ubuntu at least); I have muscle memory to use dpkg -S `which $foo` to figure out which package a binary is, but that doesn't work if dpkg thinks the binary is in /bin (e.g. ping), since it'll ask dpkg who installed /usr/bin/ping, which is nobody.

It is small fiddly things like this all over people's packaging and personal scripts that break.