Edit: This glitch is fixed in 1.20.
Before I detail how this glitch can be triggered, I will first mention that well-designed levels never rely on a glitch to complete the level. This glitch can be abused to prevent a level from being completed by players who are unfamiliar with the glitch. I believe it is important to detail how to trigger this glitch so you know how to remove it if it inadvertently ends up in one of your levels.
When resizing a level from the Start/Goal bar, the level resizes half a screen at a time. However, you can tap on the arrows at the base of the flag pole to resize a level one square at a time.
The condition for the glitch begins when a level is resized so there is an equal number of screens and squares after the final screen. For example, if the level is four whole screens plus four squares long, the glitch will be triggered. This can easily be tested by making the main area be one screen plus one square. This must be done before the sub area is created.
Add in a warp pipe and drag Mario into the pipe to create a sub area. It is important that the main area be resized first, because the sub area is initially the same size as the main area. The sub area can only be resized by the Start/Goal bar, which resizes half a screen at a time.
As long as the sub area is not resized, the final column will be glitched. Blocks can be drawn in this column, but cannot be moved onto the column. Pipes and enemies cannot be placed onto this final column. Most importantly, the final column is intangible, which will cause a problem in your level if the player jumps over an enemy expecting to stop against the wall, only to fall through.
This creates an avenue for adding a special item, such as a 1-Up, that a player thinks cannot be reached if they are unaware of the glitch. However, this is poor level design, and has the negative side-effect that the player can lose a life by falling through the wall. It’s best to avoid this glitch when making well-designed levels.
This glitch was discovered in Super Mario Maker version 1.10. It was fixed in 1.20.