There is a particular thought that shows up right after a slip, and it is worth naming because it is sneakier than most. It is not "I want another cigarette." It is something closer to: I already smoked one, the damage is done, I might as well finish the pack and start fresh tomorrow. That thought feels almost rational in the moment. It is actually a classic sunk-cost trap, borrowed from the same faulty logic that keeps people watching a bad movie because they already sat through half of it.
The flaw is in treating cigarettes like they come out of a single, already-spent budget. One slip does not use up a fixed allowance that the rest of the pack is somehow entitled to draw from. Each cigarette is its own separate decision, with its own separate cost to your lungs, your cravings, and your progress. Smoking a second one because you already smoked a first one is not damage control. It is choosing to spend twice, based on a story that the first spend already happened, so the second one is free.
This trap is subtly different from the more famous "last cigarette" myth. That one is about the story you tell before you smoke, a plan dressed up as an ending. The sunk-cost trap shows up after, when a slip has already happened and the mind is looking for a reason the next one does not really count. Both lead to the same place, but they need to be caught at different moments.
The way to interrupt it is almost embarrassingly simple: treat the cigarette you just had as a closed, separate event, and treat the next decision as a brand new one, unconnected to it. One cigarette is a slip. A pack is a relapse. The gap between those two outcomes is entirely decided by the very next choice you make, not by anything that already happened.
If you have slipped, you have not lost the quit. You have made one decision that did not go the way you wanted. The next one is still yours to make, and it is not obligated to follow the last one anywhere.