The issues, as I see them are as follows:
First, everyone’s carbohydrate tolerance is different. Not everyone needs to limit themselves to 20 g/day of carbohydrate, but we advise that as the limit, because it works for nearly everybody, and we want people to be successful when they try this way of eating. If you have a bit more carbohydrate than 20 g on a given day, but you are still under your personal threshold, mazel tov, you’re fine.
If the amount of carbohydrate you eat takes you over your personal carb threshold, whatever it might be (it depends on how insulin-resistant you are), then you’ve got some problems. First, excess glucose in the bloodstream causes damage. That’s why insulin mobilises to get it out of the bloodstream and into muscles and fat tissues where it’s safe (of course it also can go into your liver and pancreas, where too much can cause problems). The elevated insulin in your bloodstream resulting from the excess glucose also causes damage.
Now, the damage isn’t going to be too great if this is a rare occurrence, but a lot of people are addicted to carbohydrate, and one slip can lead to more carbohydrate and end up undoing your ketogenic diet. And the ketogenic diet is a lot like showering, in that it brings a lot of benefits if you do it regularly, but all those benefits go away if you stop doing it. That’s totally unfair, I know, but you’ll have to complain to the Big Guy upstairs for setting things up that way.
If you are still in the initial phase of going keto and haven’t reached fat-adaptation yet, well, you will probably set yourself back by several days if you exceed your carb threshold, which may or may not be worth it. If you are fully fat-adapted, you might consider whether exceeding your threshold is a good idea. After all, you have gone through quite a bit of work to persuade your muscles to reactivate all those fatty-acid burning pathways, heal the mitochondria, make new mitochondria, and so forth, so is it really worth it to suddenly force your muscles back into glucose metabolism? This is, of course, in addition to the risks of glycating your haemoglobin, reactivating your systemic inflammation, raising your blood pressure, stiffening your arteries, increasing your cancer risk, and all the other consequences of elevated serum glucose and serum insulin.
If, once you are fully fat-adapted, you want to experiment with just where your carb limit lies, that’s one thing, but if you are being seduced by the remembered taste of some carbohydrate-laden treat, that’s another. I generally find that, whether I get kicked out of ketosis or not, extra carbohydrate makes me feel pretty bad the next day—all my aches and pains return, I get bloated, and all those other consequences that make me feel it really isn’t worth it after all.