The first movie I will ruin for you is: The Robot vs. The Aztec Mummy
Evil Dr. Krupp (or more commonly known as 'The Bat') builds a cardboard box robot to steal a treasure from the Aztec Mummy's tomb. The robot gets into a shoving match with the Aztec mummy for maybe a minute at the very very end - after the movie builds up to this advertised clash - and the movie kind of just stops - Mummy wins!
Until you realize the robot spun off his own sequels!!