as the title says: if you have an open trapdoor overtop a channel - when water flows beneath it, the trapdoor closes- i guess the flowing water updates the blocks
to reproduce:
dig a 1 wide ditch - at least 3 long.
place a trapdoor over the ditch
place a bucket of water in the ditch away from the trapdoor
watch it close.
reported by /u/PE_crafter at reddit.com/r/mcpe
This happens on the iPod 4G. I think this is meant to happen according to the block update. Mojang could put a fix out for this.
EDIT: If there is no canal and only 1 block under the trapdoor. You can put water in the hole and it will close directly after. Works with lava too.