Trapdoors have their texture shifted to the other side when they are placed a direction facing East or West, but not North or South. Also, when a redstone signal is sent to the Trapdoor, it initially will not react. After a few pulses, it changes its default position to the normally powered state.

To reproduce:

1. Simply place a trapdoor facing East or West.
2. Send a few pulses of redstone to the trapdoor.
3. Cut off the redstone signal.