Placing a trapdoor of any kind (with no other blocks touching it) on the side of a block and giving it a redstone current will cause it to spawn a dropped trapdoor of its kind.
Environment
Windows 8.1
Processor: AMD A4- 5300 APU with Radeon HD Graphics - 3.40 GHz
RAM: 6.00(5.45 usable)
System Type: 64-bit
This issue has been reported already. It has been linked to the original bugreport.
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