We all know that when an aircraft has a lot of maintenance done on it that the plane will end up at the gate with nonstandard positions on switches and maybe a few circuit breakers pulled and forgotten, especially the TO warning CB annoying them everytime they add power. Pressurization left in manual, TO warning horn CB's pulled, dozens of other things too. Always look at everything when taking an aircraft maintenance has been working on. They have a different checklist than we do and sometimes don't follow that one. I had one experience when I got a 737 in Burbank in the afternoon, flew it until late at night, got the same AC the next morning, did the normal crew oxygen test and when I pushed the flow switch oxygen pressure dropped to zero. Just before I picked up the AC the previous crew at a down station had the bottle replaced because of marginal pressure. Even though it showed normal pressure when I got it, it was trapped pressure because the valve was off. Had to take a delay because the bottle was in the fwd baggage compartment and I couldn't get to it because of bags. Chief pilot asked me why I didn't write it up giving maintenance the delay because maintenance said it was our problem because it wasn't written up. Asked him how much the FAA would fine us knowing the bottle was installed noon the day before and we flew it all day until the preflight checks the next morning with the valve shut off.