It's possible I joined a custom server with a mod installed. After I played on though one, the following 4 servers used the same physics (moon gravity, unlimited ammo, unlimited grenades, hoarde attack on extreme) and then I went to play a single player game and I had same physics working. After the first game a player said "yup l4d on turbo is always fun" so I assumed that was the name of the hack/mod... I've seen a couple youtube vids of the same mod but wasn't sure how my SP got affected, and eventually went away... I am assuming after I quit out of Steam and launched it later. Maybe when joining the server I downloaded the maps/physics and it persisted until I quit?
Either way, was ridiculously fun. On average we were getting over 10,000 kills per person per level.