The AA literature talks about a fatal progression. The way this works is this: during a drinking history, there are periods when the individual can stop for a bit, or play around with stopping for a few months, with short periods of sobriety punctuated with days or weeks of drinking. Then, with no warning, one of these micro-relapses extends and takes the person into a relapse that lasts for a year or a relapse the person cannot come back from.
The reason why it's never safe to have one drink if you're an alcoholic is that there is a progression taking place, which has both linear and step elements. Gradually, the alcohol takes hold, but there are step-changes, too. It's like jumping up and down on a floor for months, then suddenly the floor gives way, and you find yourself on the next storey down. That next storey may be quite different than the storey above, and the loss of control may be categorically worse.
I experienced this: for years I could not drink or could drink only moderately when I was with my parents; when I hit 21, something snapped, and I could not stop myself from getting uproariously, stociously drunk even in front of them, although the consequences of this were so awful it was the last thing I wanted to do.
To sum up: when someone slips, they might come back tomorrow, or they might come back never, and you can never tell. It's no good saying that, if things really become intolerable, one will just stop again; Dr Jekyll thinks that, once he becomes Mr Hyde, his Dr Jekyll sense will bring him back, but by the time he becomes Mr Hyde, the Dr Jekyll sense is gone, and all he is left with is Mr Hyde, and Mr Hyde never wants to come back. Mr Hyde has the upper hand and doesn't give a damn.
The alarming footnote is that this applies also to immoral behaviour: a little bit of immoral behaviour perhaps seems harmless, but the truth is that it grows and grows, and the more it grows the less you feel like stopping, until it takes over altogether. Immoral behaviour (or rotten thinking) deadens the soul, thereby deadening the only part of you that could wake you up if things go too far. When the fire consumes the fire alarm and the fire extinguisher, there is no hope any longer.
This is why we ask repeatedly for God to remove all our character defects. We don't know which one will kill us if it is allowed to grow so far our soul is so deadened we drink.