What's behind the 1-2 pipe?

[Tuli0hWut]

What are you attempting to reveal with this?

I believe what he is showing is more trivia for the site. I only found trivia under Super Mario Bros. that stated world -1 is accessible through the glitch (and more negative levels are accessible through a flag hack in -1), not that the middle pipe take you to 5-1, the largest level skip in the game, and that particular level skip is only accessible through a glitch.
That being said, there's probably a lot of trivia I could have possibly missed, some trivia intertwines with other trivia, and I have not done this myself.

you gave Dazz a good explanation. thanks man.