Triangle de Pascal. No duh. 
Advanced Algebra class: had them find the expansion of (a+b)^n for n=0, 1, 2, and 3. We wrote down coefficients in a triangular manor, and only 1 out of 24 students figured out that Pascal's triangle was being formed even though they had all been taught it the previous year. It's interesting to see what students retain from one year to the next.