Unpowered inserters pick up moving items on a belt. This only happens for items passing by, not when placing an inserter on an already full belt.
[2.0.8] Unpowered inserters pick up items from a belt
Re: [2.0.8] Unpowered inserters pick up items from a belt
Thanks for the report. That is a known bug that was fixed for 2.0.9 already but is still pending a release.