How to Get Unlimited 1UP
You can get infinite lives (1UP) in Super Mario 3D World by using an exploit in the early areas of World 1-2. This allows you to go through the main game smoothly with a large amount of 1UPs collected.
Enter the pipe and run all the way forward through the level. When you reach the area with the first two Goombas near a wall with two open pipes in the background, take out the Goomba nearest to the Koopa Troopa ahead. Jump on the Koopa Troopa to strip him and carry his shell up the platform between the two pipes jutting out of the background.
Once you are on the platform, simply throw the shell down to the space in front of the pipe on the right. The shell will become trapped in the space below. Jump down on to the shell to continuously until you get as many 1UPs as you want.