To reproduce: set up a small stream of flowing water 1 block deep. Then break the blocks under all the water blocks and replace them with slabs. This should create a stream of flowing water over non-waterlogged slabs. Drop an item into the stream (I've only tested by dropping into the source block, but it may work with the flowing blocks as well).

The result: the item entity will at first appear to flow with the water, but this is only visual. It will soon return to the place it was dropped and not be pushed. But the bug is that you also can't pick it up. In fact, the Age never increases, the PickupDelay never decreases, and although it shows up with commands like "say" or "data get entity", it will not be teleported with "tp" or killed with "kill". It seems to be in a weird sort of in-between state of existence.

Logging out and then back in (or perhaps unloading and loading the world? I've only tested in a single-player world) fixes it, and it becomes a normal item entity again.