US distilled spirits contain more alcohol than appears on labels
I had a problem with too high a proof going into the bottle. One product I oversaw/made, at a contracting distillery, was severely over the proper proof. The distillery owners son was told to bring to correct proof and bottle. When I tasted a batch a few weeks later during a tasting, I noticed that flavor was more pronounced than it should be. (I had tasted the small amount prior to bottling that was full strength, and that I had then brought to proper proof myself, for quality control. So I knew what it should have tasted like.) The proof in bottle was 97+, instead of 80. I assume the kid hadn't done the dilution calculations correctly. I wanted to pull all bottles and reproof. Well that didn't happen. No one else noticed because the product was so well distilled, so smooth, that no one besides a top spirits expert, or distiller would notice. Except that they got more drunk than usual.
From the TTB BAM (http://www.ttb.gov/spirits/bam.shtml) > Mandatory Label Information > Alcohol Content > Tolerances
* 0.25% alcohol by volume for spirits containing solids in excess of 600 mg per 100 ml
* 0.25% alcohol by volume for spirits bottled in 50 ml and 100 ml containers
* 0.15% alcohol by volume for all other spirits
For our cocktail bitters, we get allowable tolerances back from TTB of +/- 2.5 - 3% on a nominally 40% abv product.