Place a dispenser, a hopper filling it and a double chest feeding the hopper. Fill everything with mincarts, so we can get quick stacks updates, as I beleive this is part of the bug, if you happen to click while it's being fed).

I was picking up items as they were being fed in the dispenser and all of a sudden, then entire dispenser was empty and my inventory had the items instead

It seems to happen more frequently when I release the shift key while spam-clicking to pick up the items. Reproducible in my map, can include map if needed, since the contraption is simple, didn't feel the need to include it.

Note: I actually found this rather useful, I'd rather see it implemented as a feature: "If you double-shift-click on an item, all items of that type are send to the container"...