When creating a sub area, be aware of the length of the level. This is especially important when a level has only one entrance/exit for the sub area.
A sign of the sub area being too wide is when you approach the return warp pipe, and find the camera keeps panning.
A poor solution is to fill in the gap with blocks.
This may be necessary when a level has multiple separate sections in the sub area, but should never be used when there is only one section in the sub area.
The proper solution is to resize the level to only the maximum length required.
This is important to remember, because when you first drag Mario into a warp pipe, Super Mario Maker will set the sub area to the main area’s length, and position you at the same location into the sub area as you were into the main area. Move to the far left, resize the sub area length, and fill in your sub area. When the proper level length is used, the camera will not pan beyond the edge of the sub area.