Will this ruin my diet ?

So I've been on a diet since sunday . I've been eating pretty healthy. 3 servings of fruit , 2 servings of veggies per day ect. Today for dinner I had a salad and afterwards I had a piece of cake. The piece was as big as 1/4 of the palm of my hand. I'm not planning to do this again (atleast not for now) but will this ruin my diet ?