If you place a trapdoor on the top side of a block, and then place a block next to the trapdoor, the trapdoor will be activated.
Video: http://www.youtube.com/watch?v=bi40jL88E8M
MC-1459
Trapdoor Bug
Resolved
Duplicate
0
Ice Trailer
(Unassigned)
Unconfirmed
Minecraft 1.4.1, Minecraft 1.4.2
Retrieved
Duplicate of MC-218, this is fixed in the next version.