I hope we all know the answer to that.
You can smoke and say it didn't mean anything and go on pretending it didn't happen
when you do that and don't start over (you've already given yourself permission once so it's easier to use the first slip to justify the second.)
you've most likely already subconscously given yourself permission to do it again.
When there is a cost for losing your quit (starting over) you are much more likely to not do it again.