It is impressive what authors can do in such a short format (many in the 4-7 pages range).

I particularly liked:

At the Bend of the Trail by Manly Wade Wellman (plants are people too)

The Desert Lich by Frank Belknap Long (never violate the Qoran)

The Epiphany of Death by Clark Ashton Smith (no good deed goes unpunished)

In the Vault by H.P. Lovecraft (beware windblown doors)

The Outsider by H.P. Lovecraft (the unnamed feasts of Nitokris)

Schizo Jimmie by Fritz Lieber (revenge is a dish best served cold)

Shrapnel by Karl Edward Wagner (there will be hell to pay)

Wild Grapes by August Derleth (beware vengeful plants)