I think this has been there since counter trading went in. There's a weird situation where the other person asks for something like 3 wood, and I try to counter offer with just 1 wood because that's all I have, and for some reason, the trading dialog won't let me send back that offer. I can only offer the 1 wood when they change the trade up and are asking for something completely different (say wheat), I can then can offer them the single wood (instead of wheat).
[something like that] I'll try and screenshot it next time it happens. I've had it happen 3-4 times. Maybe I'm just not paying attention and something proper is going on, but I think there's a bug here.