When Hopper Minecarts are actively picking up items from an above container while you place items in it, some of your items will disappear.

To set it up:

1. Place a Hopper Minecart with a chest above it
2. Place a stack of items in the chest
3. Now place another stack of items directly in the Hopper Minecart while the other items are being pulled in from above

Expected result:

When all items are finished moving into the Hopper Minecart, I would expect two stacks of items (given the example above).

Actual result:

Less than two stacks of items will remain in the Hopper Minecart. In some cases, half a stack will be missing.