There are certain Rush maps where it is impossible for the Attackers to NOT blow up some of the crates. Can't think of any specificily off the top of my head(just got home from work so I'm not in that mind set just yet) anyways there are some maps where the defenders never skunk the attackers until a couple of M-Coms into the map. At which point it becomes impossible. I've never ever ever lost the last two M-coms on Siene Crossing, but I've never ever ever beaten the defenders on the first two M-coms either