Yoshi null sprite in mouth - Normal and SA-1 comparison
Maybe it's already well known but I found this.
SA-1 give you wings!
In any case, SA-1 or not, don't use the normal Yoshi sprite in level where you can keep items in mouth and despawn/respawn him. Use Yoshi blocks.
The shell getting destroyed is not related to this glitch. It's just that when you put two shells next to each other sometimes one can hit the second.