For those of you who weren't around yet during the first Class Updates, this was a 'fake' update created by someone from the community.
He would basically reveal parts of the upgrade every day, just like they did during the 'real' valve updates.
http://www.samuelpires.com.br/tf2/engineerupdate/day01.htmlI believe there was another one as well, but can't find it..
Had something to do with a chicken launcher and a mini sentry that would follow you around, both flying and SWIMMING when you were under water.