When a city expands to border an unimproved wild improvement, attempting to build the improvement will take up the city's build queue.
If this isn't classified as a bug, it ought to be. This is incredibly stupid, as literally the only effect is to require the player to temporarily tetris their city placement to avoid connecting until after the improvement is finished. Hooking them up afterward is usually trivial and entirely beneficial, so this mechanic/bug doesn't actually discourage chaining to improvements. It just makes for an unintuitive, immersion-breaking, and almost entirely meaningless alteration to the order in which you place buildings. Well, unless you don't understand the mechanic, in which case you just get penalized for doing the logical thing. This needs fixing.