Bug

[KloePoek]

you can replace current terrain improvement within the city borders of someone you are at peace with.. so you can start mining your opponents worker factory, or worse start planting forests all over his area

 

[Dark Sheer]

Actually this is so that you can help your Ally. Of course it works both way, you can use this to backstab your ally as well

 

[Psychlone]

Yeah, one of my friends does this, signs right of passage agreements with his neighbors, then goes and plants forests all around their cities to starve them down and weaken them. It's pretty cheesy.