Some apps can prevent taking screenshots, like my banking application does. You'll get a "Couldn't capture screenshot. Storage may be in use"-errorwarning. Now my phone (i9300 / JB) is rooted, I think that somehow we could overcome this application limitation. I did search for this error on XDA, but nothing was specific to apps preventing screenshots, so that's why I'm opening a new Q. For the record, I can save regular screenshots just fine with power + home combo.