A slip is a slip, one drink and you are not on the path of sobriety. Back to day 1. That's my opinion of it at least.
Now, if you slip and have just one and can stop yourself there, pat yourself on the back and don't think of it as an excuse to keep going. You slipped but you pulled out of it. Don't slip and slide....that was only fun with water when we were kids! Slipping and sliding with booze is way too dangerous.
In summery, yes, 1 drink is back to day 1 but it's not an excuse to "live it up" and finish the bottle because you'll only hate yourself more in the morning. Spoken from fresh experience.