1. They run out of food only 2/3 of the way through, when they had enough to last till the end.
2. They compete in the reward Auction. Part way through, Tina & Elis finally realize they can do better by pooling their resources. If they'd all done that from the beginning, they would have been able to buy more food (by paying less for each item), or if it ran out before the money did, at least had a few bucks to take home.
3. They build a new camp on a slope! The first thing you learn when pitching a tent (the first time it rains) is to put it on level ground. Maybe you could excuse the dry river bed (very common in Oz), but any slope would have left them wet.
4. They take out Nick. The Invisible Man dropped out of the Kucha alliance, had no more motivation and would probably have voted for himself next time. The Big Three should have used the opportunity to break up Elis & Roger's solid alliance.

Big ##### (Hatch) must be tearing his hair out. If he could somehow have been there incognito, it would have been an easy million.