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