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When 2 inserters with stack bonus are chained together they can become deadlocked when handling single stack items. This only happens to single stack items and seems to be related to the fact they are placed on the ground if not actively wanted by anything by inserters. Whereas stackable items are held in hand by the inserter.


Setup
In the below screenshot ignore the lack of power, that's just to get things positioned. Step 1: have an inserter with stack bonus try to unload multiple single stack items sequentially into the same tile. Seen in row 1 and 2 in image
Step 2: add a second inserter with stack bonus to try and pick up from the tile with the items placed on it. It will wait indefinitely.


Example Save game
Attached is a save game example of the bug in action. Base game with no mods to reproduce in 16.20.
The deadlock can be seen occurring in the save game below. It also seems to be possible to occur if the first inserter is running faster than the second one. In this example save game the deadlock occurred after 5-10 inserter load actions without external input.
To clear the deadlock pickup the items between the 2 inserters (framed by the belt pointing at it).

couldn't reproduce this. the second inserter just grabs the item from the first, then lastly grabs the item off the ground.
[edit]ok i see, it only happens with artillery shells.

quick video made showing it attached, had to zip the mp4 to get it uploaded.

Oops say your edit after doing reply.
It seems to happen to any single stack count entity as I originally hit when using the universal pallet mod with loading & unloading pallets which are single stack items.

yeah, stack size seems to be messing with the inserter stack logic.

happens with stack sizes of 10 (infinity chest) with a stack inserter with full bonus.

Thanks for the report. It's now fixed for the next version of 0.16.