Factorio Forums

Drills place items between 2 lanes of a belt. If the belt direction is directly away from the drill, the item will typically immediately slide to the left or up. If this spot is occupied by an item, both items will move in the direction of the belt clumped into a slow-moving pile.

To replicate:
Build a drill that outputs on a T-section belt moving items away from it. Assuming drill points south: place items on the upper lane of the left belt of the T-section. If the item enters the T-section immediately before drill places its output, the pair of items will get "glued".

This will result in clogged belts only occasionally, but is problematic to troubleshoot if you don't know what to look for.

The same can be done with inserters and that costs me in my last game a lot of resource until I checked, that the items blocks...

I need to update this:
https://forums.factorio.com/wiki/inde ... king_items