It's not really missing, but you won't find the May 23rd, 1987 cartoon in any of the collections (although you will find it in The Calvin and Hobbes Essential Treasury, on page 253). For completeness of my index, here are the topics referenced in this cartoon:
To see the cartoon, visit http://images.ucomics.com/comics/ch/1987/ch870523.gif.
P.S. For the Calvin & Hobbes offical site, visit the Calvin and Hobbes official site.