Downward floor clip is usually considered impossible in Super Mario Bros (NTSC version). This one is a rare exception.
DaSmileKat came up with the idea of using a vine and enemies to do a floor clip. It requires at least 2 enemies in SMB1, and 1 in SMB2J because of higher bounce. But it's hard to think of a place to actually perform it without using hacks or memory editing, so I came up with the idea of showing it in the beginning of 1-2, with a vine "stolen" from 5-2 by Luigi's death.
This glitch requires extremely precise Y subspeed and Y subpixel. The vine is used because it provides a different Y subspeed than jumping and sliding, and the 2 Goombas are used to further adjust Y subspeed until we get the perfect Y subspeed required for this ground clip.
There hasn't been a completely new SMB glitch found in 2 years, because rare glitches like this are extremely hard to find. Thanks to DaSmileKat for this. :)