Overating during diet?

I am currently eating max. 1300 calories but today I overate by approx 200 calories because I accidentally counted my calories wrong (1520 calories now which is my maintenance but for me it's like weight gain) I'm so scared that I ruined my diet and I admit I'm afraid of gaining weight. I can't excercise it off because I'm busy with work so I can't excercise. I wish I could undo it. Anyone got ideas?
Overating during diet?
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