As the others have said, don't beat yourself up about it and don't give up because of it - it's just one small blip and won't do much harm diet wise but could do more harm if you stress over it. It could knock you out of ketosis as the body now has some carbs to use up, but if you get straight back on the wagon you'll soon get back into ketosis again, it's not like at the beginning when it takes 3-5 days. You may feel hungry later, but stay focussed, keep drinking the water and you'll soon be back where you were. One ham sandwich will not do much damage to your weight loss, as the amount of cals is not that horrendous, but you may retain water for a day or two if you WI is coming up.
Keep your chin up and get straight back on board :hug99: