Steps to recreate:

1. Build a two-block thick wall.
2. Place a piston one block above the ground on the end of that wall, oriented so the piston will extend along the shortest edge of the wall. Place more wall underneath the piston.
3. Place a hopper so that it will be pushed and pulled by the piston. Make sure to leave an air gap underneath the hopper.
4. Power the piston, then crouch and stand so you are underneath the lip of the hopper while touching the wall.
5. Unpower the sticky piston.

If done correctly, the hopper will drag you along with it, causing you to clip through the wall.