The only way for nembutal to fail is if you don't take enough of it, or if it's impure.
Nembutal used to be a sleeping drug and still is a type of anesthesia in veterinary use.
If you were to take enough to not die you would probably just wake up 24h-48h later since it acts as intended, I would speculate that you would be rather thirsty and hungry, anecdotal reports say they felt like they had a really bad hangover. Nembutal is not a stimulant, so overdosing won't fuck you up if you were to survive. However, 12 grams from AV and there is no way you will wake up, 840 out of 840 people die from 12 grams, not one failure been reported under administration.