Factorio Forums

Not sure if it's reproducible in 0.18, but in the latest 0.17, if you are playing with someone and, while close by, you click on them to open their inventory and trade, they can walk away while still being able to trade. I was able to give my friend items from over a dozen chunks away.

Being able to open another player is an admin-only feature and can be done from any distance and with the /open *name* command.