The alternate strip (corresponding article details significance in syndication) of November 28, 1985. The original was never reprinted, whether in the first printed collection, the first treasury or even The Complete Calvin and Hobbes; all of these feature the alternate strip instead.
The strip for May 23, 1987 was reprinted in The Essential Calvin and Hobbes and The Complete Calvin and Hobbes, but was missing in Something Under the Bed Is Drooling.
In the strip, Calvin drops a teabag into a glass full of ice cubes, hoping to make iced tea.
Alternate Format strip
The Alternate format strip was a strip from 1988/08/28. It was originally printed horizontally in newspapers, but was redrawn vertically for the books. It was not reprinted in the original format until Calvin and Hobbes: Sunday Pages 1985-1995. The original version is included in The Complete Calvin and Hobbes.
Christmas Eve poem
The Calvin and Hobbes Christmas Eve poem was a strip from 1989/12/24. There were two different versions of it. The first was printed horizontally in newspapers, and the floor that Calvin and Hobbes were sleeping on was red. The version that was printed in books was vertical and had them sleeping on a green floor.