In Excel 2007, worksheets can have up to 16,384 columns (column XFD) and 1,048,576 rows - that might seem excessive but mega-Excel users were regularly coming up against the current 256 column / 65,536 row limit.
Thanks to the nice guys at Office Watch for that one.
What they didn't comment on was what one does when a student accidentally hits Ctrl and the down arrow, types something, or worse - hits the spacebar - then realises his or her mistake and either scrolls (Level 1 students), uses Ctrl+Home (Level 2 students) or puts the cell reference they meant to go to in the name box and hit Enter (Level 3 students) forgetting to erase whatever data went in row 65536 and wonders why so many sheets are coming out of the printer.
Row 1048576??!! Just never, ever let them put borders on the whole sheet, folks, OK?!