We were faced with the following scenario:
My character with an alchemist follower and a mule is carrying his maximum of eight items, and is being pursued by two other characters. During an opponents turn, he casts mesmerize on me, and chooses to steal the mule. This means I am now carrying four more items than I am allowed, and must therefore leave four items on the space I am on. Rather than doing this, I instead choose to turn four items into gold using the alchemist follower, in order to deny the other players my loot.
This started an argument, since they say I can only use the alchemist skill on stuff in my possession. I argued that it is not until I move away from the square, leaving the four items, that they leave my possession. I also point out that the fact that I get to choose which four object I leave, rather than them being drawn from my hand at random, shows that until I have made that choice, they are clearly in my possession, and as such available for the alchemist to turn into gold.
They argue that the taking of the mule means the stuff on it immediately drops on the floor, and since I cannot encounter the space without landing on it again, I cannot turn it into gold.