When two hoppers are placed on top of each other, one is puling an item at the same time another deposits an item (resulting in two items being moved simultaneously). This bug has, for me, persisted across worlds and client restarts.
The primary reason to take issue with this is the fact that it breaks the classic storage system (look at Wattles Item sorter https://www.youtube.com/watch?v=ruw1elYIQo4) or any tech channel expansion of that. When two items are pulled, the hoppers don't lock properly.
In regards to a clip, I have one testing all the above claims but it is >10MB.
Link to the clip of the issue
https://drive.google.com/open?id=1Lxs0N-j2X4RuxASGdjP-DQn_WY9w93ba
Please provide a screenshot showcasing the issue, or upload the video somewhere else and link to it here so that we can understand what the issue is.