if a stack of an item is put into a hopper facing downwards into another hopper via the hopper GUI or dropping the stack of items onto the hopper; then both hoppers will move an item down at the same time. this will cause a build up in the next hopper facing sideways unless the sideways hopper is facing into another container and has another hopper below it. should the sideways hopper be facing into a container and have a hopper below then the items will be distributed evenly between the container and the hopper below.

Notes

• only happens if given a stack of an item type (given 2 different items at the same time will not cause this)
• will not occur if the items come out of a container (e.g chest or dropper on top or dropper putting items in hopper)
• will not occur if items are given 1 at a time
• does not happen with non stack able items