Yes, the Flat Cap. I am trying on making my own new custom power up, and so far this is what I got. What it does is flattens Mario for a short amount of time so he may be able to walk under very short entrances that are way to small for him to go threw walking or crouching.     

Original Video - More videos at TinyPic Now, I haven't actually gotten it replace a cap, or atleast add one (I doubt you can add one) but that's the basis of it.