The game crashed on level 99 - you have a memory leak some where the exception was the system was out of Heap, would have copied it but java window crashed as well.

How do you get it to move? I think if you made the movements on the joints vary depending on how much weight it was currently holding it would help.

The collision detection is pretty good, there is this scenario http://www.greenfoot.org/scenarios/9908 that has its source available for download, it is a pixel perfect detection scenario. In my opinion programming technique is secondary to game concept and story writing. :D

If you publish the source - there is nothing stopping someone from re-uploading the scenario, of course doing this can get that specific user banned.

That can happen when your Internet connection gets a bit flakey or if you are accessing greenfoot site through a VPN, usually when it happens the scenario takes a long time to load.