Last-Modified doesn't even have to be a valid date string:

http://www.nikcub.com/posts/persistant-and-unblockable-cooki...

As an update to that post from 2011, I never did get the browsers to update their parsers.

This E-Tag bug has been known for over a decade. As part of my post I tried to find the earliest written record of it and found a post in 2003:

http://www.arctic.org/~dean/tracking-without-cookies.html

It seems to get 'rediscovered' at least a couple of times a year.