Twins

This puzzle appeared in Microsoft Intern PuzzleDay 2004, whose theme was urban legends. The puzzle has two pages that look nearly identical. We put both pages in teams' puzzle packets, about ten puzzles apart, without telling them they were two pages of the same puzzle. Teams had to figure this out for themselves.

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Hints

The two pages of this puzzle have nine subtle differences between them.
If you extend the grid lines by drawing horizontal and vertical lines through the drawing, you will notice that each of the differences lies within a single "square" of the grid. The column and row headings for each difference can be combined to form a letter and an offset, e.g., Y +13. This can be converted to a letter by indexing into the alphabet. For example, Y +13 gives L because if you go 13 letters past Y in the alphabet (wrapping around the end of the alphabet if necessary), you reach the letter L.