When I'm reviewing, I find that I often get into a groove where I'm hitting every character, one after another, moving very quickly. This causes me two problems.
1) When I try to recall certain characters outside of reviewing, it is considerably harder. There is a degree to which all my RTK stories are linked in my mind, so the simple act of sitting down to review them is a great memory aid. When I fail to recall a character outside of reviewing, I'd still like to be able to move it to the Failed box here, so that I can give it some extra attention and make sure the connotations of my story are correct.
2) Sometimes, when moving quickly through my expired stack, I will be absolutely sure I remember a character, write it down, and then hit Space-Y before the character's image has really registered in my head. On some occasions, I realize a moment too late that I confused it with a similar keyword (most recently this happened with 掃 and 洗). I'd like to be able to manually go back and mark these failed as well, since these are often the stories that need the most refinement.
I'm not sure how big a problem this is for other people, but for me it would be nice to have a "Fail" button on the study page, so that I could go back and manually put kanji into my failed box in these situations. I can also imagine that this would be useful if there are some RTK kanji you are simply not interested in learning - you could then manually remove these from your reviewing cycle.