Mario & Luigi: Bowser's Inside Story
Mario & Luigi: Bowser's Inside Story
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There is an unused graphic of an earlier version of the water fountain from Toad Town, with a statue of Princess Peach as a mermaid.
Contributed by MeleeWaluigi
In Mario & Luigi: Bowser's Inside Story, there are quite a few unused rooms without anything in them, most of them used for testing. One of the rooms appears to be a pipe in Dimble Woods leading into bowser's body that can only be accessed through the use of hacking. It was probably removed because once you've found all the attack pieces there you had no need to go back. Another one of those rooms are in Dimble Woods, along with what appears to be the Tower Of Yikk on the ground. This might suggest that originally bowser was supposed to fight it in dimble woods, but it was replaced with Bowser's Castle.
Contributed by Spinjitsuninja
Upon meeting Broque Monsieur as Mario and Luigi at his shop in Dimble Wood, he says that he has met them somewhere before but can't remember. This is probably a reference to when Bowser first got the Vacuum Block and Broque saw Mario and Luigi inside of Bowser.

Monsieur does not like that the brothers hit the ? blocks around the kingdom and decrease their "value" and refuses to sell them anything.
Contributed by jllmprrt
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Going to Bowser's Castle Spoiler:as Mario and Luigi and going down a blue pipe in the concession stand area will lead you to an underground room. Entering the correct code in the following room will Spoiler:open a vault, and entering the vault one finds a large icy room. Hitting the button with a hammer will cause some of the ice to melt, revealing three Spoiler: Shroob enemies from the previous installment in the series, Mario and Luigi: Partners in Time.

After the battle, Starlow finds out that these creatures once invaded the Mushroom Kingdom, and that Mario and Luigi Spoiler: defeated them as babies. Starlow also finds out that Spoiler: Luigi defeated the Shroobs by crying in the past, another nod to Partners in Time . This almost assures that the Mario and Luigi in Bowser's Inside Story are Spoiler: the babies in the previous game.
Contributed by jllmprrt
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There is an unused item that looks similar to a Life Shroom, but is named 'Dummy'. If used, Mario (or Luigi) will throw it in the air (while inside Bowser) and it will explode, healing 60 HP back to Bowser. It was most likely scrapped because of the fact only the Sea Pipe Statue and Durmite can damage Bowser from inside.
Contributed by CuriousUserX90