While building a sorting machine i discovered, that items could get stuck on top of hoppers while floating on water, if they are not moving at the edge of the block. In the test setup i used hoppers beside each other and flowing water on top of them for the transport. The test items were sent from a dropper, which is behind the dirt wall. Depending on the horizontal position of the test items (looking from picture 1), they get stuck while moving through the middle.
I tested this behaviour with other configurations and got the same result.

EDIT: I forgot to mention, that all slots of every hopper are taken by items of an different kind. So ideally all test items should arrive at the end. It seems like there is a hollow on top of the hopper where the items could collide with.

Could be possible that i missed, that this behaviour is intended. Sorry in that case.