Q2: You went right through the piranha plants, why didn't you die?
A2: Super Mario Bros. has many problems with hit detection, making it possible to jump through the piranha plants with accurate timing.
Q4: You went through the firebar in level 8-4 and did not die, why?
A4: Same as piranha plants. it just works. Poor hitbox detection.
*Note The hitboxes in this game are weird. List of things you can get hit by in certain spots and not die:
- Tops of piranha plants
- Ends of firebars
- Bowser's/Hammer Bros axes