After a bad day (or week)
"I blew it at the wedding / on holiday. Is it all gone?" No. Here is the arithmetic, and then the one thing to do.
The arithmetic of one day
A kilogram of body fat is on the order of seven thousand kilocalories. A genuinely enormous day — say four thousand kilocalories over what you used — is therefore roughly half a kilogram of fat, and most "bad days" are nowhere near that. Set against a month of a moderate deficit, one such day costs a few days of progress. It does not cost the month, and it cannot cost the year.
The scale the next morning will say otherwise, and it is wrong about what it is measuring: the extra is water bound to salt and to the glycogen refilled by a day of carbohydrate — about three grams of water per gram of glycogen — plus what is still in your gut. It leaves over the following two or three days on its own. Do not act on that number.
A deficit is an average
The deficit that removes fat is the average over weeks, not any single day's. One day above maintenance lowers the weekly average a little; it does not reset it. What does undo a week more often than the bad day itself is the response to it: fasting the next day, cutting the target in half, "earning it back" with a punishing session. Those produce hunger, a second slip, and the cycle people recognise as "I always fail on Mondays". The bad day was a rounding error. The compensation is the pattern.
The only action
The next meal, as planned. Not smaller, not skipped. That is the entire correction, and it works precisely because it is boring.
When a bad day is a pattern
If it keeps happening — the same evening, the same trigger, every week — the problem is the plan, not the person. A plan that cannot survive a Friday is too tight, has too little protein, or leaves no room for the things you actually enjoy. The what to eat step and the hunger page are where to adjust it. A plan that works is one you can slip on without falling off.
In the app
The streak counts a day on which you logged something, not a day on which you hit every meal, and nothing in the interface punishes a day over target. That is a deliberate design choice, for the reasons on this page.