Continue on to Act 2 without acquiring three Mutation bars in Act 1 for a certain character. Acquire at least two Mutation bars in Act 2 then go back to an Act 1 level and complete it on a higher difficulty for a 100% level completion. You will be given a Mutation bar, but you will not be allowed to add it because you can only add three bars to each character in Act 1. The game will not let you past this screen or regress any of the bars. You will have to restart the game.