If you spam left/right while being next to the sides, you will eventually get the piece stuck in the wall. Works on both sides of the wall for both players.

e.g. https://gyazo.com/31d65b3cb1c6978df37123ee5e4333c9

Reproduction steps:

1. Get a L piece
2. Move it to the walls as close as possible
3. Spam the direction change keys
4. Lose the game because you got outplayed by a superior opponent