Edit: This glitch is fixed in 1.20.
The original Super Mario Bros. introduced the idea of an underwater sub areas, spanning no more than four screens in length. It’s an idea worth using if you would like to make an underwater area, but do not want to design an entire underwater level.
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.
Floating pipes first appeared in Super Mario Bros., in World 5-1 and World 6-2. These appearances set the initial foundation for a floating pipe. The Lost Levels further defined their use. Following the rules set by these games will improve the appearance of floating (and hanging) pipes in levels designed using Super Mario Bros. mode.