When Yoshi is clipped into the wall, he will start rising up till he reaches the top solid block or will keep going up off screen. This is mostly used as an obstacle to over come to get Yoshi through but can also be used as a troll to take Yoshi away if you do not get him fast enough.
This is gltich tech.
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Dismounting Yoshi into a stack of coins and hitting a P Switch will cause Yoshi to climb up and be usable on the other side. On/Off Blocks will also work. Turn block bridges will not cause Yoshi to go up but rather he will fall right through.