In the Media Library, I was trying to crop out whitespace from a small image--and was frustrated that the crop button was disabled. After some experimentation and Googling, I realized that it would not allow me to crop unless I was cropping to at least 150 pixels high or 150 pixels wide (my thumbnail size is 150 x 150).
Apparently this is a feature, not a bug. It is frustrating, though, when I would like to quickly make a change in a small image instead of editing it locally in a graphic editor and uploading it again. So my question is: is there any way to change this behavior so it will allow images to be cropped to any size?