If an item spoils while in hand, then inserter will hold an item that is known to be a spoil product of a different item. Such items get slightly different treatment when trying to insert into entity where the item does not fit into input inventory. If its attempted to be inserted into assembler ...

The use case is this:

https://www.reddit.com/r/factorio/comme ... k_newbish/

I'm not the OP, I have the same issue.