At the end of the level in 1-2 you will come to a pipe that leads you to the end-of-the level flagpole, instead of going through the pipe you can glitch through the top of it. In World 1-2 there is a glitch that will take you either to World -1 or you can use it to get to World 5-1. Each one will take you to the world of the corresponding number, the options will be world 2, 3, or 4. Ride a platform all the way up till you can jump onto the blocks at the top of the level and walk right till you approach three pipes. These particular platforms will be moving in the up direction instead of moving downwards like the previous one.
While you're in World 1 - 2, head toward the end of the level where you will see the second set of moving platforms. This allows you to skip to world 2, 3, or 4. If done correctly, you will be rewarded with continuous 1-ups if you're able to keep going. The timing here must be perfect or you will end up sending the turtle flying. Hit the second turtle to make him stop coming down the stairs and keep jumping so your character doesn't hit the ground.
At the end of the level, you will see two turtles coming down the stairs that lead to the flagpole. To get unlimited 1-ups, go to world 3-1 and head towards the end of the level.
