I offered the trader 4 gold rings, which it moved down to the bottom window, I then proceeded to offer it rope, picking the rope one by one from my inventory and offering it to him, which he would accept and move down to the bottom window.
I then decided to offer more than one rope at a time, re-selecting the ones already offered, and dragging all the rope from my inventory to the trader window when the client crashed with the following:
EDIT: So this happened again after restarting the client. I traded with the same trader, this time offering it a different item than before. I got up without completing the trade to make coffee (you know, wurm and coffee), when I got back, same error.
I had offered it the item, it accepted. I left, came back, time passed, error happened. All of this without ever completing the trade.