Inserters can be placed on other players in multiplayer as a clients

[Sgt_Imalas]

When playing in multiplayer, if you are not the host, you can place inserters on top of the host (potentially other players too).
Trying to place it on yourself will mark the area as blocked.
The host is not able to do that

[Sgt_Imalas]

Edit.
Upon further testing, this is not valid; the bounding box of inserters being smaller than a tile just made it look that way