If you are engaged in shopping with someone (be it a vending machine or NPC) and sell items - putting more items into the 'Sell' windows than you have free inventory slots yourself, and then cancel the trade.. guess what? Your inventory only receives the items it can fit, the rest are lost. No 'Overflow' window appearing either. I only lost some non-vital items, but other players might well lose valuable items this way. Something to be fixed over the course of time.