It wont happen if you have a limit for the challenge. When I want to participate in such contest/challenge, I have to set my limit. I need to predict how much is the possible reward I'll get if I complete the challenge and I will not spend more than the possible reward (max 50%). Lets say the possible reward is $100, I will try it with $50 only but I will also set a limit on one other thing. If I have a luck with my session but I have not reach the challenge, I'll stop to hunt the challenge.