I love this show to death but sometimes I can't help but think it's rigged. They almost have to, it'd be pretty boring if one team didn't finish in time so the other guys ran the challenge alone.

The episode that hardened my belief in this was the "Walkers" episode, where teams had to build legged machines that would carry the whole team along a course.
One team took an approach using hydraulic cylinders, which by principle is complex and dangerous. They ran into many construction and engineering problems along the way. By the time the sun fell, they hadn't even really shown the finished hydraulic control box, a sophisticated and vital system of hoses and valves. No one had ever mentioned or shown how they were turning the engine's (which never started) power into hydraulic power.
Yet somehow the next day, all this is figured out, and they even learned how to operate the thing.

My favorite episode ever was "Amphibious Vehicles" mentioned by Kefabi. The violin playing in the background as the losing team sunk was priceless.