I made aunderground water channel where the water flows 7 blocks before falling one block down. Where the water falls, there are 2 blocks where the cannel is 3 blocks high. when I walk  downwarts in this channel, with pressing z, When I collide withe top block after the place where the water falls, In place of going down and continuing my way down, a strange up and down movement starts wich let me get stucked behind the top block. this movement stops when I press shift or go back. Also loook at the video I made.