Haha, I knew one of you folks were gonna realise this the hard way.
The MyUni timetable system is changing all the time, depending on lectures/tutes/labs numbers currently in their database.. which is dictated by you guys (many thousands of you) changing timetables around the clock. So the only way for you to definitely be able to change your timetable and then change it back to exactly what it was is if you were the only one who changed their timetable during that time, which obviously is pretty impossible. The longer you leave it before trying to change back, the less chance you have of succeeding since the database would change more as time passes.
Moral of the story is: If you're happy with your timetable, and you wanna see if you can go even better.. don't just 'Choose this timetable' to preview it.. otherwise chances are you'll get screwed.
