Hitchhikeing on Flar’s post:
When all the variables lead to a final gravity that is lower than the advertised final gravity, and a brewer bottles when he hits that advertised final gravity, the beer will still be fermenting - and producing CO2 from the remaining sugar in the original beer as well as from the carbonating sugar. The CO2 pressure can exceed what the bottles can contain and the bottles burst explosively.
The beer makes a sticky mess. And, if you are in the area of the explosion, the flying glass can injure you.
Yes, I did. It took two sutures to close the cut in the back of my arm.
So, if you can’t trust an advertised final gravity, what can you do? Take a tentative final gravity reading, wait two or three days and take another tentative final gravity reading. If the two readings are the same, that is (almost certainly) the real final gravity reading.
Occasionally brewers experience a stuck fermentation. Usually in that case the reading will be noticeably above the anticipated/advertised final gravity. Search the forum for actions you can take to restart fermentation, but don’t bottle until you’ve solved the problem - fermentation can restart.
Another (better) alternative is to do a Fast Ferment test. I leave my first gravity test sitting out at room temperature to complete fermentation. When the gravity stops dropping, I consider that reading to be my final gravity. A more rigorous approach is to add LOTS of yeast to the sample and wait until the gravity stops dropping. I use the simpler method because I use kegs which can tolerate higher pressures than bottles.