This game was originally intended as a Nintendo GameCube game. By summer of 2007, Super Paper Mario was moved to Wii. An early version of the game had a different looking version of the Whoa Zone from Chapter 4. Also, an early screenshot shows Mario under the effects of the Mega Star, but is colored very differently (giving his current colors rather than the initial ones seen in Super Mario Bros.). Oddly, he is shown in his proper colors in the official trailer that was released at the same time, which may imply that the screenshot’s release way simply delayed.
In a screenshot featuring Big Blooper, there’s an odd Pixl that is shaped like a ladder. It never actually appears in the final game. This could be an early version of Carrie. Also, in the same screenshot, Tippi is present, but she was kidnapped by Francis at this point in the game, and by the time she is rescued, also Big Blooper cannot be fought again. Finally, the scene depicting Bowser and Peach’s wedding at the beginning of the game was also slightly different in beta versions; the ceremony would appear to have once taken place in more of a room than an open area, there were no flowers decorating the columns, the rising of the Chaos Heart was surprisingly light, and Count Bleck raised his cloak up earlier than he did in the final version. Plus, in the screenshot, Peach is standing instead of having been knocked over, and Bowser looks normal than shocked. [Info from Mariowiki]