My consultant always says if you have a blip just draw a line under it and get back to food optimising the next day. Food optimising means HEXB, HEXB, Free Food AND syns. She's very strict on this, and says that if you're not having syns then you're not food optimising. Half the plan isn't The Plan.
Just shows how much opinions can differ. Touch wood I've not had to make that decision (yet). I went over syns once accidentally (product confusion) but didn't notice til a few days later, when I'd been having all my syns anyway. Luckily I got away with maintaining that week and no actual gain. Phew.