1/2 a scone, in my opinion, will do nothing to sabotage you. You have to live!!!!! I have one off plan thing a week, I normally plan my lapse for a Sunday, its a piece of cake, a chocolate bar, a sausage roll - whatever. I lose 1 - 4 pounds a week, so I must be doing something right.
The trick is, don't let something naughty that you eat dent your confidence, that's when you are likely to really binge and do some damage.