I just ran through the demo and everything was great. Although, I believe I just dumped 400 uBTC into a black hole (was given 500, donated 100, didn't bother saving the wallet #).

I'm sure a lot of people will do exactly the same. You might want to not give out the 500 uBTC so easily / fast, I doubt much of that will ever be used.

Another solution is to update the donate screen to be donating all 500 of it, that way it's completely used up by the end of the demo.