I make a lot of these, from the small, to the big. This time, I'm telling the whole interweb, just so I remember and can prevent it next time:
I. Printed 36 copies of the old Algebra 1 benchmark 1 test, instead of the new one. Only found out when a student noticed that the number of bubbles on the OARS scantron didn't match the number of questions on the test.
PREVENT: Double-check the hell out when setting up materials for class.
Paper is expensive, and we only get two reams each month from the school. Also: it's a pain in the butt to correct scantron tests by hand.