From Minecraft Parkour Wiki
    • Prior to 1.8, the player couldn't step onto some blocks which had a ceiling above (bug report).
      • To fix that, an alternative method was added to the stepping procedure (visualized in the video)
    • Prior to 1.8.1, stepping could be used to glitch into the floor.
      • This is because the floor below the player's bounding box is never considered for collisions during step-assist, but the procedure could force the player to move below floor level anyway (the bounding box was lowered by 0.6b regardless of the amount it was elevated by).
      • It's even easier to do in 1.8.0, as you don't need a ceiling directly overhead.
    • The current implementation of stepping is still somewhat flawed.