If you are not familiar, Evernote is a free application for your computer as well as mobile devices (especially great on the iPad) that lets you organize notes of any sort (text, picture clips, websites, audio...) and store them on the cloud, i.e. making them accessible from anywhere and synchronizing automatically across all your devices.
I have found it priceless as an
error log, much more useful than a spreadsheet. I use pdf versions of the study books on my iPad and sometimes computer and with Evernote the magic is that you can clip the actual question off the pdf file and save it under its number, and then i keep a second notebook with the answers. This way, when I want to go through my
error log, I simply pull up the notebook and solve the questions, without having to look for them anywhere.