I remember on world 8, when I finally beat Bowser Jr., I was like "YEAH! SUCK IT!" then that bridge formed.... yeah. Guess what I was thinking then.
I finally beat Bowser after more than 15 tries.\
Just so some of you know, before you go into Bowsers room, on the right side of the door, next to the wall, is an invisible block, holding a fire flower. If you hit Bowser 25 times with a fireball, he'll die. Or you can go under him when he jumps. It's hard though, you have to stay close to him at all times, and you'll know he's going to jump when he bends over.
Hope that helped.