Bukkit/Bukkit — pull request #750 by Peter200lx (2013-01-06 03:58:28+00:00)
Title: Support inverted TrapDoor (BUKKIT-3390)

Similar to Steps and Stairs, TrapDoors now have the ability to be inverted controlled by the fourth bit (0x08)
http://www.minecraftwiki.net/wiki/Data_values#Trapdoors

Feature added in Minecraft 1.4.2
http://www.minecraftwiki.net/wiki/Version_history#1.4.2

Trapdoors
Can now be placed on the top half of blocks [34]
Placement works similar to that of slabs and stairs

Added logic pulled from Step.java and Stairs.java

Associated issue: https://bukkit.atlassian.net/browse/BUKKIT-3390