Don't think one bit of bread will make a difference to your weight but it will probably knock you out of ketosis (packed with carbs).
If you have any ketostiks I'd test yourself tomorrow morning to see if you've been lucky.
oh iv been so tempted by having a huge chunk of bread and butter, but knew i would feel guilty after! dont worry though, everyone makes mistakes, just dont get into the mindset that you've ruined the diet and go and eat loads, just accept it and carry on like normal, and rememeber the guilty feeling in future when you want to snack xx