Debian "sid" users beware of the dpkg 1.16.2 upload

But realistically for this to happen, it means that either have you have removed (without purging) the native version of the package while it wasn't multi-arch aware yet and you later installed a foreign version of the package once it has been converted to multi-arch.

Or you had only a foreign version of the package installed (before its conversion to multi-arch), that you removed and now you installed the native one after its conversion.

Because as soon as you have two versions co-installed at a given time, it means that the field was present.

Anyway, I just reproduced it here (need squeeze + wheezy in sources.list):
$ sudo apt-get install ftplib3:i386=3.1-1-8
$ sudo dpkg -r ftplib3:i386
$ sudo apt-get install ftplib3:amd64=3.1.1-9
$ ~/cleanup-multiarch.pl
INFO: Foreign package detected: ftplib3:amd64 (Multi-Arch: same)
PROBLEM: one instance of ftplib3 is not Multi-Arch: same
SOLUTION: dpkg -P ftplib3:i386
[...]
PROBLEM: ftplib3 has missing info files
SOLUTION: apt-get --reinstall install ftplib3

So the two problems are real even if relatively unlikely on a typical usage of multiarch.

Again thank you, at least I could test that my script was working as expected :-)