Context: I'm on a diet and I ate three chocolate bars yesterday.
Speaker 1: I'm supposed to be on a diet, but I couldn't help myself and I ate three chocolate bars.
Speaker 2: it happens we have to reward ourselves sometimes. dark chocolate is actually good for you! 
Speaker 1: But I'm supposed to be on a diet! I can't lose weight if I have no self control :(
Speaker 2: hey it was just one day! you got this... work extra hard for the next 5 days! 