A simple one for starters.

The Hippo $1 commemorative was issued in the Year of the Happy Goose, in the wake of Unseen Academicals. These stamps appear to have had two printings (at least), with one of them being quite paler in both the brown shades of the illustration and the olive greens, even the pale green background to the entire sheet. Below is a composite to show the differences - click on it to enlarge.