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I found a glitch a long time ago and posted it here, and I decided I might as well post it again because some people might find it interesting.
Basically you just need a Mega Mushroom in your inventory. This glitch is easier to do after beating the game, so you can restock on Mega Mushrooms whenever you please. Keep the mushoom in your inventory and go to any fortress that has a Bowser Jr. battle without him throwing shells. World 1's fortress would probably be quickest.
Immediately as you start the fight with Bowser Jr., activate your Mega Mushroom, but don't use it to defeat him. Let him run into you, or run into him, so that you knock him over. Now stand right next to him so that you keep knocking him over whenever he tries to get up. Stay there until you shrink down again. Now, you may try to attack him. However, when you jump on him, he will not take damage; instead he will simply fall back over as he had when you were knocking him down with the Mega Mushroom, and the sound of Mega Mario destroying something such as blocks will play. If you jump on him again while he is on his back, you will only bounce off of him and do no damage. The only way you can damage him by ground-pounding.
This gitch will still remain active even if you get hit, only ending if you defeat Bowser Jr. or lose a life, or exit the stage any other way.
I first found it when I was bored and decided to mess around with Mega Mushrooms, and I took one to one of the towers. I went into the fight and just used it to knock Bowser Jr. over instead of beating him with it just because, then afterwards when I went to actually fight him all that just happened. I just put two and two together and tried it in other towers and it kept working.