As an example of the unintentional: I went to a very good friend's for dinner. Because he is a good friend, I felt able to tell him about low carbing, and I even spelt out in an e-mail what I couldn't eat.
This is the (lovely) meal we were served:
Appetisers - tortilla chips, a whole, baked Camembert (to share) with bread sticks to dip in
OK, I could have avoided this; I managed to avoid the tortillas, but the cheese was to die for. I 'only' had 2 breadsticks
Starter - leek and potato soup with melba toast - served dished up to us
by now I'm heading rapidly into 'in for a penny, in for a pound' territory
Main course - barbecued/marinated lamb, with a honey crust, mashed swede and potato, red cabbage with cranberries, carrots, broccoli
so only the broccoli was truly safe here!
Pudding - Christmas pudding with white sauce and/or rhubarb crumble with custard and cream
I don't like Christmas pudding so I had the crumble, because I love rhubarb, and there was no cheese board because we'd had cheese as the appetiser!
The only saving grace was the lack of alcohol for me because I was driving!
The point of this is to say that it happens. But we're in this for the long haul, and so in the great scheme of things, as long as you return to low carbing immediately afterwards, it will all even itself out.