When standing on the edge of a block which has a height of about 0.5 block, with water stream adjacent to that block, and cobblestone wall/moss stone wall/fence of any kind / closed fence gate of any kind in front of the player, he/she is constantly jumping without pressing the jump, or any other button at all. To reproduce the bug follow these steps:

1. Place a water stream that is one block wide, and surround it with blocks
2. At the end of the water stream and right next to it on either side place cobblestone/moss stone wall/ fence/closed fence gates of any kind
3. Place a half-block adjacent to the block placed last (tested with these blocks so far: any type of slabs, cake, bed, lower side of any type of stairs, and 5 layers of snow cover)
4.Stand on the edge of the half-block and the water stream
5.Have fun jumping while afk!

EDIT: I have seen a similar bug report, but in that one the block in front of the player was not a fence/wall. MC-8959