That's OK with me. With any of my scenarios, if I have the "Open In Greenfoot" Button there, you can use the code however you want, in whatever you want (count that as permission for stuff like challenges).

Just a random guess, I haven't looked at your code at all, but maybe, while the barrel isn't rendered (if you even do that), it resets it's place when rendered (seems like it might do that), but it expects the player to be in the first place it was rendered, the spawn point. So it moves weird because it thinks the player is in a different spot. If that is the case, you'd have to do stuff in the code where when the barrel is rendered, it makes sure the player is in the right spot. Like a said, a random guess, but it came across my mind last night, and it might be it.